One of Brian Miller's Garden Tractor Pulling Tips & Tricks

10-18hp Kohler (or Carter [model N]) CarburetorInformation about the Kohler Carburetor, Various Fuels and Fuel Systems

Identification of Carter and Kohler Carburetors -

Kohler (or Carter [model N], that was used only on the early 10hp engines) made three types of old-style carburetors for their K-series single cylinder 10hp through 18hp engines. One is a size "26" which is used on their 10hp (model K241), 12hp (model K301) and smaller twin cylinder engines. It has a 26 millimeter or 1.07" diameter throttle bore and flows 98 CFM (cubic feet of air per minute) with the venturi removed. The next is a rare one. It has a 28 millimeter or 1.17" diameter throttle bore and flows 100 CFM with the venturi removed. It's used on the 14hp and 16hp engines. The third is a size "30" that's also used on the 14hp (model K321), 16hp (model K341), 18hp OHV (model K361) and larger twin cylinder engines. It has a 30 millimeter or 1.200" diameter throttle bore and flows 110 CFM with the venturi removed.

By the way - numbers stamped on a carburetor flange or anywhere else mean nothing to me or to Kohler Engine dealers. They're just random numbers that only the Kohler factory know the meaning of.

The #30 carburetor came on the 14hp, 16hp and 18hp OHV engines, but the carburetor that originally came on the 18hp OHV engine has a larger venturi in it, while the 14hp and 16hp carburetors have a smaller venturi. That's the only difference between the two. Other than that, the carburetors are the same. The throttle bore in both carburetors measures 1.2". The carburetors that's used on the 18hp OHV single cylinder engine and some larger [flathead] twin cylinder engines have a larger diameter venturi, but the same size throttle bore, which is 1.200".

The venturi in a stock [10hp and 12hp] #26 carburetor measures .808" in diameter, and flows 74 CFM. With the venturi removed (having a straight-through 1.07" throttle bore), this carburetor will flow 98 CFM, which is about 1.3 times or 65% more air. The venturi in a stock [14hp and 16hp] #30 carburetor measures .920" in diameter, and flows 84 CFM. With the venturi removed (having a straight-through 1.2" throttle bore), it'll flow 110 CFM, which is about 1.3 times or 77% more air. And a bored-out #26 carburetor, will out-perform a stock (non-bored) #30 carburetor. And a #26 Carter or Kohler carburetor cannot be bored large enough to match the size of a #30 carburetor. The venturi "wall" is simply too thin.

NOTE: Boring of the venturi in the carburetor alone won't help to increase engine performance. To increase the overall airflow in and out of the combustion chamber, and if sanctioning club's rules allow this, the intake (and exhaust) port runners will need to be enlarged, too. This is called "porting an engine." The [stock size] valves will also need to be reworked for improved airflow. Or larger valves will need to be installed.

If you have a 14hp Kohler engine, and you can't find a #28 or #30 carburetor, then a #26 carburetor will work great on a 14hp engine for ordinary yard use. Personally, I ran a #26 on the 14hp engine that's on my 6,000 lb. self-propelled pulling sled for 10 years and the engine didn't lack any power at all.

For pulling competition, carburetor design and size depends on port sizes, valve sizes, if the intake valve and seat have 30°/31° angles and undercut heads, and the cam profile (lift and duration) is important. If an engine has stock ports, valves and cam, a plain stock carburetor can be used. But if the ports and valves have been reworked for more airflow, and a bigger cam is going to be used, then a bored-out carburetor with a 1" (.995" NQS legal) venturi can be used for best performance.

The size number on all Carter [model N] and Kohler carburetors is embossed inside the carburetor and can be seen by looking inside the upper part of the choke end, just above the venturi. (Click the picture to the right for details.) A "26" carburetor has a 26 millimeter diameter throttle bore, which converts closely to 1.024 inches (26 ÷ 25.4 = 1.024"), but actually measures 1.07". A "28" carburetor has a 28 millimeter diameter throttle bore, which converts to 1.102 inches (28 ÷ 25.4 = 1.102"), but actually measures 1.17". And a style "30" carburetor has a 30 millimeter diameter throttle bore, which converts to 1.181 inches (30 ÷ 25.4 = 1.181"), but actually measures 1.200".

But if there's no number, you'll need to accurately measure the diameter of the throttle bore to determine the size carburetor you have.

The 26mm carburetor obviously has a smaller throttle bore. Therefore, is restricted as to how much air can flow through it, even with the venturi removed. It'll work excellent on a hot 10hp or mild 12hp engine, but the larger 28mm and 30mm carburetors flows more air, 7.7% more for the 28mm, and 15.5% more for the 30mm. Which are ideal for a hot 12hp and larger engines with porting, polishing, bigger valves and a big cam. If you feel that using a single 30mm carburetor isn't enough for your particular engine, then either a "Super Carb," Dellorto, Mikuni, S&S Super D or a twin Kohler carburetor setup (on a "Y" intake) should be used.

Identification of Walbro Carburetors -

The Late model Walbro carburetor - good for non-competition pulling applications onlyWalbro carburetor originally come on all Magnum engines, and are very good, reliable and they last a long time before requiring service. The only problem with Walbros if you want to use one on a pulling tractor or for high performance application is they can't be bored out or modified in any way because the throttle bore wall (where the venturi is located) is too thin. Therefore, they're limited to how much air they can flow. The US government told Kohler and all other small engine manufacturers to redesign the carburetor that's used on their engines so they'll produce less air pollution. So they did away with the old style Kohler carburetor and started using the redesigned emissions-controlled Walbro carburetor. The older Walbros have an adjustable high speed main jet, which is located on the lower side of the carburetor. But the newer ones have a fixed or non-adjustable high speed main jet. This is to keep people from setting the air/fuel ratio too rich and create more air pollution.

Walbro carburetors work great for ordinary lawn and garden equipment, general yard work and they're perfect for basic stock low rpm/low-performance pulling tractors with an engine that's governed to a maximum of 3,200 rpm with a fixed main jet, or at 3,600 rpm with an adjustable main jet. A carburetor with a fixed high speed main jet is limited to just 3,200 rpm and should never be operated faster for an extended length of time. Although the venturi cannot be bored out on a Walbro, they work equally as well as the older Carter or Kohler carburetors when used on a stock engine. The main jet in the newer Walbro carburetors are calibrated at the factory to provide the engine with just the right amount of fuel at 3,200 rpm. Therefore, the fixed jet inside the carburetor cannot be replaced with an adjustable one.

Walbro carburetors having a fixed main jet are designed with emission controls in mind to run leaner to create less air pollution. However, the main problem with most Walbros having a fixed main jet is during cooler weather operation. During cool weather, and running at 3,200 rpm, the engine will sometimes run too lean on fuel and operate erratically. To fix this, simply enlarge the hole in the main jet a few thousands of an inch. The factory main jet hole size for 100% gasoline is 3/64" (.047"). If necessary, enlarge the main jet and fuel inlet holes with a #55 (.052") drill bit. That's a .0052" difference. After doing this, for cool weather, the engine should run better, last longer and produce more power. But don't enlarge the main jet hole too much! A too big of a hole will cause an engine to run too rich on fuel, making it blow black smoke out the exhaust and possibly fouling the spark plug. If the hole is made too large, the carburetor will be useless for gasoline, and E-85 fuel must be used. But if an engine is going to be used only during warm weather and if it runs okay [during warm weather], don't enlarge the main jet hole.

On the newer carburetors with a fixed main jet, the main jet hole may need to be enlarged .002"-.003" to give the engine a little more fuel so it'll run better at higher rpms. Otherwise, the engine could "burn up" or wear out prematurely due to the lean air/fuel mixture. I have an assortment of tiny drill bits that I use on my customer's lawn & garden equipment carburetors.

IMPORTANT! A new style Walbro carburetor with a fixed high speed main jet should never be used on a non-governed (wide-open throttle) engine or on an engine that's going to turn at 3,200 rpm! The reason for this is because if an engine operates faster with a carburetor having a fixed high speed main jet, it'll run too lean on fuel at high rpm (it'll draw much more air than fuel), which will cause the combustion chamber to overheat and the excessive heat will likely damage the piston and rings and possibly warp the cylinder head and/or exhaust valve or even crack the cast iron engine block. A lean fuel mixture can also melt away part of the aluminum in the combustion chamber of an aluminum block engine.

The size number on all Walbro carburetors is embossed inside the carburetor and can be seen by looking inside the upper part of the choke end, just above the venturi. A "52" carburetor has a 26 millimeter diameter throttle bore, which converts closely to 1.024 inches (26 ÷ 25.4 = 1.024"), but actually measures 1.07".

And being the old-style 7hp (K161) and 8hp (K181) Carter model N carburetor venturi can't be bored out either, the only option for pulling competitively with these is to fabricate an adapter to fasten to the intake port on the engine block and use a #52 Walbro or #26 Carter or Kohler carburetor.

The best way to determine which engine any particular carburetor is made for is to measure the diameter of the throttle bore. Because a set of numbers that's stamped on the carburetor body mean nothing, not even to Kohler Engine dealers. If a carburetor measures .822" (13/16"), then it's made for the 7hp or 8hp engines. If it measures 1.07", then it's for the 10hp or 12hp engines. And if it measures 1.17" or 1.2", it's for the 14hp, 16hp (flathead) or 18hp OHV engines.

If the threads strip out where the float bowl retaining bolt goes, to fix this, cut about 1/8" off the bottom of the protruding part and reinstall the float bowl and retaining bolt. It'll tighten the bowl slightly more onto the carburetor, and you'll need to check and see if the two side fuel feed holes aren't blocked or partially blocked with the retaining bolt installed and tightened. If they are blocked, they can be redrilled so the fuel will reach the main jet. This works the same with an OEM retaining bolt and an aftermarket bottom adjuster. Be sure to back the adjuster out before installing so the needle won't jam into the main nozzle and get damaged. And if the threads strips again, you may have to get another carburetor body. Because I believe the metal is too thin for welding or a Heli-Coil insert.

How to Set the Governor Adjustment - Top of page

If your engine revs with no closing of the throttle shaft, or surges, then the governor is probably out of adjustment. To set the governor on a Kohler (or virtually any engine)...

  1. Install all the throttle linkages, governor parts, etc., in their respective places.
  2. Loosen the bolt that clamps the lever to the governor cross-shaft that protrudes out of the engine block.
  3. With the throttle plate blocked in the wide open position, rotate the cross-shaft counterclockwise until it stops. Now tighten the bolt.
  4. That's all that's to it! But if this doesn't fix the problem, then perhaps a governor part inside the engine is damaged or the screws came out of the throttle plate, where it fastens to the throttle shaft.
  5. You can also go here for further details and detailed pictures: http://cubfaq.com/govadjust.html.

IMPORTANT! Do not mistakenly rotate the cross shaft clockwise (opposite rotation than what it shows in the drawing above) until it stops, tighten the clamp and then run the engine! Doing this could cause the lever (see below) to jam into the governor flyweights, breaking off the lever and possibly destroying the governor gear assembly.

If the governor cross-shaft (the shaft that protrudes from the block) gets bent due to rough handling of the engine, and if it's not bent too bad, just use a hammer to straighten it. It's made of mild steel. But if it's bent severely and breaks off, it must be replaced. If this happens, usually the bushing/nut (Kohler part # 235476) will break and it will need to be replaced, too. Or, if the above adjustment was attempted, and the governor shaft keeps rotating without stopping, this means the flat lever on the shaft has broken off. This will allow an engine to operate dangerously at wide open throttle at all times.

To fix either of the above, another governor shaft (Kohler part # A23525601S) must be installed. To install another shaft, the entire engine must be completely disassembled. This means EVERYTHING inside the engine block (crankcase) will need to be removed. And then the replacement shaft can installed from inside the crankcase. But before the old shaft is removed, the bronze bushing/nut on the outside must first be removed. The governor gear assembly doesn't have to be removed. The shaft lifts out from inside the crankcase and is installed in reverse order of removal. And it'll be a good idea to place a small bead of weld on the flat lever to secure it to the shaft to prevent future breakage. (I think this is something that Kohler should have done.) I realize that this is a lot of work just to replace a small [important] part, but it must be done in this way. There is no other way to replace it.

How to adjust the fuel mixture screws on a Kohler (or virtually any early small engine) carburetor - Top of page

First of all, on the Carter and Kohler carburetors, the hole towards the bottom of the long high speed main needle adjuster is supposed to be open, so the engine can draw fuel for idling from the float bowl. The idle fuel passageways for a Carter or Kohler carburetor goes as follows: The fuel is first drawn through the high speed needle adjuster via the lower hole towards the bottom, then it travels upward, then over and down toward the idle mixture screw (which sets at an angle), and then the fuel is drawn into the throttle bore, next to the throttle plate. The information below is mainly for Carter and Kohler carburetors. The fuel mixture screws are used as follows:

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If your engine blows black smoke out the exhaust while running, then here are the causes:

Black exhaust smoke is when an engine is getting too much gas and/or not enough air in the combustion chamber. The causes for this are either... (listed from most common to rare causes)

  1. Choke plate isn't opening all the way.
  2. Too rich of fuel mixture adjustment on the carburetor.
  3. Choke plate isn't fully opening.
  4. Worn float valve (needle and seat).
  5. Bad float.
  6. Float out of adjustment. Set the float so it's level with the carburetor body with the carburetor turned upside-down.
  7. If the baffle shield is old and stiff, it can push down on the float after the float bowl is installed, preventing the float from shutting the fuel off at the float valve (needle and seat).
  8. Too much fuel pressure if using an electric fuel pump.
  9. Hole in the main nozzle (jet) have been drilled too large.

Also, sometimes flooding of the carburetor will deposit gas in the motor oil, but only in a vertical shaft engine. Because gas can't seep into the crankcase on horizontal shaft engines. The cause of the problem is the carburetor is flooding. The gas seeps past the piston ring gaps and into the crankcase. To fix this, clean the float valve (needle and seat) in the carburetor, or install a new carburetor kit. Also, clean out the fuel lines with compressed air and install a new fuel filter. And then the oil will need to be drained and fresh motor oil installed.

Causes of Carburetor Flooding -

What prevents most carburetor and fuel system from having problems when using gasoline is to run the engine out of fuel or draining the entire fuel system if it's not going to be used for more than 30 days at a time. DO NOT TRUST FUEL STABILIZER! If there's no fuel left in the system, then there should be no problems. The following causes are listed from most common to rare.

  1. If fuel drips out of the carburetor...
    1. Dirt, debris, lint or a hair lodged in the float valve and seat or a damaged fuel filter. This will prevent the fuel inlet valve (also known as the needle and seat or float valve and seat) from fully closing in it's seat. Cleaning of the valve and seat, flushing out the fuel line and new replacement of the fuel filter is required.
    2. A worn or damaged fuel inlet valve and/or seat. New replacement is required.
    3. Loose float valve seat (that's threaded in the carburetor body). Sometimes these will loosen, resulting in a light flooding condition. If tightening it doesn't stop the fuel leak, then a new float valve and seat will need to be installed.
    4. Use a medium size hammer to flatten the center of the float bowl from the inside so when installed and the bottom screw is tightened, the bowl will press tighter around the O ring and baffle shield to prevent a fuel leak.
  2. If gas constantly pours out of the carburetor...
    1. Check that the float valve is installed correctly. The rubber tip goes in the seat and away from the float.
    2. Bad or damaged float (hole in float). To test for a bad float, if it's a hollow brass float, remove it from the carburetor and shake it next to your ear. If you hear a sloshing sound inside it, then it has gas in it and new replacement is required. You can also feel if there's gas inside it by shaking it. Repairing by re-soldering a brass float will most likely not work or last long.
    3. Float stuck to the bottom of the float bowl. This is caused by engine setting in storage for a long period of time with gas left in the gas tank and carburetor. To prevent this from happening or happening in the future, drain the entire fuel system, and then loosen the float bowl retaining bolt to finish draining all the fuel from the carburetor. Cleaning of the float and bowl is required.
    4. Float bowl has a small hole in the bottom of it. This happens to aluminum float bowls and is caused by corrosion from water being in the fuel system. Water is heavier than gas. So it goes below the gas and straight to the carburetor. During winter, the water will freeze in the carburetor and crush the float. New replacement is required.
    5. If the carburetor on an engine have been setting for some time without an air filter housing, and if gas pours out of the carburetor only when cranking the engine, especially with a mechanical fuel pump, then the gas stops pouring when the engine isn't cranked, check for a clogged atmospheric vent hole. Sometimes mud daubers (wasp-like insect) will build their nest in places like this.
    6. If an electric fuel pump is being used, it either has too much pressure, or if there's a fuel pressure regulator in-line, it's set too high or stuck in position.

If your engine revs up fine, but won't idle slowly, or dies as it idles down, this means that either the carburetor mounting flange is warped, creating a vacuum leak, the idle circuit or idle fuel passageways are clogged or there may be insufficient lifter to valve stem clearance. This is how to fix these things:

  1. First check if the carburetor mounting flange is warped. Use a belt or disc sander or a piece of sandpaper placed on a flat surface and rub the carburetor back and forth on it. If the flange is warped, it'll make contact where the bolt holes are, but not in the center.
  2. Next, check for a loose or worn throttle shaft. You can tell if it's worn by gently moving it side to side by hand. Click here to learn how to fix a worn throttle shaft.
  3. If the throttle shaft feels snug, then the next thing to check is the high speed adjustment needle valve. Check to see if it's open by sucking air through it with the pointed end in your mouth. (This is the only sure method I found that works best.) If it's clogged, then it needs to be replaced.
  4. If the high speed adjustment needle is open, then the last thing to check is the very small hole behind the welch (expansion) plug that's located on the side of the carburetor body. In most cases, the welch (expansion) plug won't have to be removed. If an engine won't idle, then chances are the hole(s) underneath the welch plug are clogged with old, dried gas and need to be cleared. But first to check the condition of the holes, remove the idle adjustment screw and then spray some WD-40 or equivalent into the [idle screw] hole and then use compressed air to lightly force the fluid out of the small hole(s) that's inside the throttle bore, next to the throttle plate. If no fluid comes out, this means the hole(s) underneath the welch plug are clogged and the plug will need to be removed with a small, pointed chisel and small hammer (be very careful doing this as to not to destroy the carburetor body), and then the hole(s) can be cleared with an acetylene welding tip cleaner (available at most welding supply stores) or a very small drill bit to clear out the dried gas and/or dirt from the main nozzle hole. Be careful not to enlarge the hole, or the engine will run rich on fuel! Then use compressed air to clear the debris from the nozzle hole.
    IMPORTANT! When using a welding tip cleaner or small drill bit, use one that just fits! Don't force it through the jet, or the hole will be made too large and the engine will run rich on fuel.
  5. After the hole(s) are open and cleared, cut some short 1/8" NPT (pipe) threads into the welch plug counterbore hole (with an 1/8" NPT bottoming tap) and then install an 1/8" NPT Allen pipe plug. Because it is nearly impossible to try to install a new welch plug and make it seal.
  6. As the last resort, sometimes the reason an engine won't idle may be in the valves. There may be insufficient lifter to valve stem clearance. In other words, it may need a professional valve job performed. Click HERE to learn how to perform a professional valve job and set the valve clearances.

If your engine hesitates to rev up upon acceleration, the following are the most likely causes: (listed from most common to rare)

If an engine starts to backfire after running for a few minutes...

How to clean the main jet in a 16hp/18hp flathead opposed twin cylinder Briggs & Stratton engine -

The main jet is located in the float bowl of the carburetor. To access and clean it...

  1. If equipped, remove the hood of the tractor (they're easy to remove).
  2. Remove the air cleaner assembly. Be careful not to drop the screws down inside the carburetor!
  3. Remove the choke cable from the carburetor.
  4. Remove the top of the carburetor.
  5. Remove the hex plug on the side of the carburetor.
  6. Using an acetylene welding tip cleaner, clean out the main jet hole. The main jet hole is accessed through the hole where the hex plug goes. And be careful not to enlarge the main jet hole, or the engine will run rich on fuel!
  7. Use compressed air to clear out the main jet hole and debris from the float bowl area.
  8. Reinstall everything in reverse order of removal.

Your engine should rev up without using the choke afterwards! This may have to be done every few years because these type of carburetors are notorious for having tiny particles of dirt that passes through the fuel filter settle at the bottom of the float bowl. The main jet is real close to the bottom of the float bowl too, which allows dirt to clog it over time.

About the Main Nozzle (or Jet) -

When rebuilding a plain, stock Carter or Kohler carburetor, there is no need to remove the main nozzle! If the main nozzle won't come out, then don't worry about it. All you have to do is make sure the bottom hole is open. But if you're modifying a carburetor to bore the venturi, then the main nozzle will need to be removed. And unfortunately, sometimes they get stuck in place and the screwdriver slot strips out. (I hate when this happens.) I found the only way to remove them is by drilling them out. Because soaking it with penetrating oil or using a small blunt punch to rotate it counterclockwise does not help at all. The process to drill it out is as follows:

  1. Firmly (but gently) clamp the carburetor with the float area upwards in a bench vise.
  2. Drill squarely into the nozzle with a 3/16" drill bit, then enlarge the hole with a 9/32" drill bit.
  3. Very gently, use a long, slender and very sharp pointed steel chisel to collapse the threaded "remains" of the nozzle. NOTE: Be extremely careful when doing this! Because the threaded aluminum tube could crack, rendering the carburetor body totally useless.
  4. Pry out the remains of the nozzle with a long needle-nose pliers. If this won't work, then use a long 3/16" diameter steel rod to drive out the remains of the nozzle through the high speed needle hole from the top side of the carburetor.
  5. Nut SplitterRedo the threads with a 3/8-24 NF tap and then install either a new or good used nozzle from a junk carburetor. If the nozzle in a junk carburetor is stuck, you can just split the threaded tube with a nut splitter. If attempting this, be careful not to damage the nozzle itself!

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If you need a new main nozzle or an adjustable high speed main fuel brass needle valve, please contact me, Brian Miller, at A-1 Miller's Small Engine & Specialty Shop (1501 West Old Plank Rd., Columbia, Missouri. 65203 | Phone: 1-573-875-4033). Please call any day between 12 noon and 8:00 p.m. Central time, and please be patient because I stutter. You can also contact me through Yahoo! Messenger: E-mail: pullingtractor@aol.com. The main jet and the high speed needle adjuster on all the Carter and Kohler #26, #28 and #30 carburetors are the same.
New brass main nozzle. Fits all Carter and Kohler #26 and #30 carburetors used on the Kohler K-series and early Magnum single cylinder 10hp-16hp flathead engines, 18hp OHV and twin cylinder cast iron flathead engines. This particular part is not available from Kohler. Professionally CNC machined in the USA. Drilled for gas. $15.00 each, plus shipping. NOTE: When rebuilding a plain, stock Carter or Kohler carburetor, there is no need to remove the main nozzle! If the main nozzle won't come out, then don't worry about it. All you have to do is make sure the bottom hole is open. But if you're modifying a carburetor to bore the venturi, then the main nozzle will need to be removed. And unfortunately, sometimes they get stuck in place and the screwdriver slot strips out. (I hate when this happens.) I found the only way to remove them is by drilling them out. Because soaking it with penetrating oil or using a small blunt punch to rotate it counterclockwise does not help at all.
New brass high speed main fuel adjuster needle valve. Fits Carter model N carburetor used on 4, 7 and 8hp Kohler single cylinder cast iron block flathead engines. Replaces Kohler part # 200410, 200410-S. Professionally CNC machined in the USA. Has lower hole enlarged and holes midway blocked. $13.00 each, plus shipping.

New brass high speed main fuel adjuster needle valve. Fits all Carter and Kohler #26, #28 and #30 carburetors used on the Kohler K-series and early Magnum single cylinder 10hp-16hp flathead engines, 18hp OHV and twin cylinder cast iron flathead engines. Replaces Kohler part # 235148, 235415. Professionally CNC machined in the USA. Has lower hole enlarged and holes midway blocked. $15.00 each, plus shipping. NOTE: Sometimes the shrink wrap on the adjustable high speed needle will allow the engine to draw too much fuel at idle. When this happens, remove the shrink wrap and the engine should idle fine.

1/8" NPT Allen tapered thread pipe plug to replace welch plug on side of #26, #28 and #30 Carter and Kohler carburetors. .25¢ each, plus shipping. Requires 1/8" NPT taper pipe tap to create threads.
1/8" NPT taper pipe tap: $6.00 each, plus shipping. Part # WT0321-005

E-85 Fuel Using E-85 Fuel in a Garden Tractor or Lawn & Garden Equipment -

What is E-85? E-85 is the term for motor fuel blends (special blend of automotive fuel) of 85% ethanol (corn alcohol AKA "moonshine") and 15% gasoline. E-85 is an alternative fuel as defined by the U.S. Department of Energy. Besides its superior performance characteristics, ethanol burns cleaner than pure gasoline; it is a completely renewable, domestic, environmentally friendly fuel that enhances the nation's economy and energy independence. The addition of ethanol boosts octane and, because it is an oxygenate, ethanol contributes to a more complete fuel combustion resulting in reduced emissions of carbon monoxide and other ozone-forming emissions. Ethanol also degrades quickly in water and, therefore, poses much less risk to the environment than an oil or gasoline spill. To learn more about E-85, visit this web site: National Ethanol Vehicle Coalition (http://www.e85fuel.com).

Fueling with E85 is not only beneficial to the environment, you'll most likely see a small increase in performance, which will be accompanied by a small decrease in fuel economy. On average, when an engine is powered by E85, the engine will have about 5% more horsepower and a 10% drop in fuel-efficiency. The added power comes from ethanol's higher octane rating (ranging from 100-105). The fuel economy decrease comes from the fact that ethanol has a lower energy content than gasoline, which means the engine will have to use more of it.

E-85 can be used in virtually any gas engine fuel system, new or old. The only problem is, the rubber-like fuel line, seals and gaskets in the fuel system may deteriorate due to the alcohol content in E-85. But if the fuel system has neoprene or synthetic rubber parts, there should be no problems. E-85 works great for pulling competition, general yard use or for any small engine equipment. It'll create less carbon build up in the combustion chamber, too. I've reworked lots of carburetors for people who use their garden tractor just to mow grass with and they love how the E-85 produces more power. If an engine is converted correctly, E-85 will cause no problems with engine wear or carburetor deterioration. Basically, all that needs to be done to the carburetor is the main jet and fuel inlet holes will need to be enlarged slightly and the ignition timing advanced slightly. If this isn't done, the engine probably won't run on E-85, or if it does, it will run too lean on fuel, possibly damaging the valves, piston and piston rings because of the excessive operating temperature.

On the 7hp and 8hp Kohler K-series and Magnum engines, with the Carter model N carburetor, the main jet hole size for 100% gasoline is .058" (#51 [drill bit]) in diameter. To convert the carburetor for E-85, both the main jet and fuel inlet holes will need to be made about 20% bigger. Enlarge both holes with a .078" (5/64" or #47) diameter drill bit. Then with the engine running at 3,600 rpm, adjust the high-speed fuel mixture needle valve so the engine will run smoothly. On the 10hp-16hp flathead single- and twin-cylinder Kohler K-series and Magnum engines with the older Carter or Kohler carburetors, the factory main jet hole size for 100% gasoline is 5/64". To convert these carburetors for E-85, the main jet hole and fuel inlet (float valve) hole will need to be made about 20% bigger. Enlarge both holes with a 3/32" (.094") or a #42 (.0935") drill bit. Then with the engine running at 3,600 rpm, adjust the high-speed fuel mixture needle valve so the engine will run smoothly. The ignition timing will need to be advanced to about 24 degrees BTDC so the engine will make full advantage and maximum power of the slower burning, higher octane fuel. No special motor oil is required with E-85. The 20% increase in the main jet and fuel inlet holes, and ignition timing applies to virtually all small engine carburetors and engines, despite the make or manufacturer.

E-85 can be purchased at select gas stations in the U.S. It cost a lot less per gallon than regular unleaded gasoline. And an engine will burn a little more fuel with E-85, but it'll also produce about 10% more power, operate cooler, have a cleaner combustion chamber and the engine will last longer.

The Advantages of Using 100% Methanol Fuel -

If the carburetor have been reworked and the ignition timing are adjusted and set correctly, methanol can produce up to 13% increase in horsepower. If the fuel is set slightly rich (which it should be), there's no need to constantly adjust the carburetor settings or ignition timing to compensate for changes in hot or cold weather conditions. Because of its low volatile octane rating (ability to burn quick), methanol shouldn't be used for general yard use. It's primarily a high-performance fuel only, and should be used only in a high-performance engine. Starting fluid or gas may need to be sprayed into the carburetor to get the engine started in cooler weather, though. And, faster gearing may be needed because of the increase in horsepower and torque.

It may take some experimentation to get a particular tractor to perform on methanol. Just because most experienced pullers get their tractors hooked-up good on it is no reason anyone else can't. Once one do get their tractor to perform on methanol, they'll be reluctant to go back to gas. By the way - methanol can be purchased at most places that sell racing fuels. And it cost less than 1/3 of racing gas fuels (per gallon).

Using Methanol Fuel in a Garden Pulling Tractor -

Methanol, also known as wood alcohol, is made primary from natural gas. And is the best high performance fuel available. Far better than any expensive "racing" petroleum fuel. It'll help any 4-stroke high performance engine produce more power and run cooler. Methanol is a safe fuel to use. But if anyone feels uncomfortable using or converting an engine for use of methanol, please consult with a professional engine builders.

Methanol will mix equally with water and it evaporates quickly. The important thing to remember when storing methanol is the keep it in an air tight container in a dry place. Because it can easily draw moisture when stored for a long period, especially in a metal container. (Unlike plastic, steel gets cold, which draws condensation.) If one suspects that water had gotten into the methanol, it can be separated easily by using a chamois. (The chamois will absorb the water and allow the methanol to pass through.)

If your club's sanctioning rules say that the methanol fuel must be able to pass a water test, then just use 100% pure methanol with no oil. (To some clubs, oil is also known as a "contaminant.") Pure methanol will remain clear in water, but turn cloudy in water when mixed with oil. And it's important that the main fuel mixture on the carburetor be richened slightly, to keep from burning the piston (because of the lack of oil in the fuel).

Methanol has no effect on rubber, neoprene or OEM carburetor or fuel system parts nor does it get stale like gas does. But it will corrode aluminum as plain water does if it is not drained and allowed to evaporate from the fuel system over an extended length of time.

Because methanol will ignite only in a narrow range of high temperatures, it is less likely to catch on fire should an accident occur. If methanol does catch on fire, water can extinguish the flames, unlike a gasoline fire.

Methanol burns much slower than the highest octane gasoline. (It has about 135 octane rating.) Therefore, the ignition timing must be advanced more than for gas when burning methanol. If the timing isn't advanced enough, some of the methanol will go unburned and little will be gained. Modifications to the point lobe on the camshaft may need to be made in order to achieve the full ignition timing setting. Methanol also requires a hotter spark. Therefore, a high-voltage/high performance ignition coil is needed, along with a wide spark plug gap (.060"). NOTE - Never run over-advanced ignition timing with any petroleum-based fuel (gas) just to try to get "more power." All that does is seriously overheat the engine and could weaken (collapse) the piston rings, shrink the piston, warp the cylinder head and exhaust valve and it could even cause the engine block to crack.

Details of the Cylinder Restraint System As methanol burns, it produces tremendous combustion chamber pressures under wide-open throttle conditions, especially in engines that's been ported, with oversized valves and a big cam, and even more so during cooler weather. To prevent possible cylinder/crankcase separation or an "engine explosion," the cylinder must be securely "strapped" or fastened to the crankcase. Especially on 12hp engines and up. So strap it now, or scrap it later! Methanol works best in a high compression engine, too.

"Strapping" the cylinder to the crankcase is when a flat piece of heavy steel or aluminum is across the cylinder head and fastened by means of two minimum 1/2" diameter threaded rods, one located just behind the flywheel and the other on the PTO end of the block. It keeps the cylinder from literately breaking loose from the crankcase because of a thin cylinder wall and/or due to extremely high compression.

Moreover, if you had a 10hp block bored for a 12hp piston, or a 12hp block bored for a 14hp piston, and you use your tractor to push snow, definitely strap the cylinder to the crankcase! Because it now has a much thinner cylinder wall, and the cold winter air is more dense (like the air is thicker or there's more of it). Dense air will build up the compression pressure within the combustion chamber, causing the engine to produce more power. But what also happens is at full throttle, this high compression is pushing upward on the cylinder head, and pulling upward on the cylinder wall. And sometimes the cylinder wall will break, ruining the whole engine. I know, I've had this happen to me before. No joke.

Methanol produces a "cooling effect" as it enters an engine at high velocity (high rpm). And, it's a clean burning fuel, it won't carbon up the combustion chamber, foul-out the spark plug or even harm the ozone layer. The crankcase motor oil may need to be changed periodically though.

Testing Methanol Fuel for Contaminants -
IMPORTANT! It's a good idea to use a small amount of special upper cylinder lubricant (such as Lucas Oil, M2 Upper Lube & Lead Additive, Marvel Mystery Oil, etc.) mixed with methanol to prevent possible piston-to-cylinder scoring and to help lubricate the valve stems and guides. If a lubricant isn't used and if methanol is ran too lean (especially in cool weather), this could ruin a good high-dollar piston and possibly score the cylinder wall. Methanol must be ran rich if no lubricant is mixed with it. But some sanctioning pulling rules (such as the NQS) don't allow lubricant to be mixed with methanol because it boosts the octane and will likely fail in a fuel test, which means the tractor will be disqualified after competing. The more oil that is added, the higher the octane will be.

When pulling with rules that require 100% methanol fuel, and if a tractor have been using methanol with a lubricant, be sure to completely drain and flush the entire fuel system. Use compressed air to clear out the fuel tank, fuel line, fuel pump, carburetor, etc. Because any trace amount of lubricant remaining will mix with the methanol and be cause for failure in a fuel test.

Methanol by itself is clear, like water. To test it for an added lubricant, pour a small amount of methanol in a clean, clear glass container with an equal amount of distilled water. If it appears "cloudy," it's contaminated. You can also smell methanol for contaminants.

Here's another thing concerning methanol fuel - when it evaporates, it leaves behind some very small particles of white, flaky calcium deposits in the fuel system. Which is normal and unavoidable. These deposits can ruin a good fuel filter, but won't harm the engine or any other part of the fuel system. So to keep from having any [future] fuel flow problems, don't install a fuel filter. Instead, filter the fuel with a paint, coffee or a fine mesh filter of some sort before it is poured into the fuel tank. Just place the filter in a funnel, and pour the fuel through it into the tank. If there are any very small particles of dirt present in the fuel, they should pass right through, being the jets have been enlarged.

Fuel filters can only filter so much of a grain of dirt. Anything smaller will pass right through, most of time causing no harm. Sometimes a hair will pass through a wire mesh fuel filter, lodging in the float valve, keeping it closing all the way because it can't get past the bend. It'll cause the carburetor to flood overnight. I've seen this happen a few times. On certain small engines, the flooding gas will seep down into the motor oil, ruining it. Briggs & Stratton riding mower engines are notorious for this. All you can do is clean out the carburetor and hope it doesn't happen again. And if there's enough fresh gas in the crankcase, sometimes the crankcase will explode if the engine backfires through the carburetor. The backfire flame will sometimes travel through the valve cover and into the crankcase.

METHANOL vs GAS -

Most garden tractor carburetors can be easily converted for methanol use. The main thing to keep in mind is that methanol requires about twice the volume of fuel than gas. (Approximately a 5-6:1 ratio for methanol versus 10-12:1 ratio for gas). So to run methanol, the main jet and fuel inlet holes will need to be enlarged to about 50% of their original size.

High compression engines naturally operate at a higher operating temperature. So when using low octane gasoline (Regular Unleaded or 87 octane rating) in a high compression engine, the octane of the gas is reduced by 1 point for every 10º above the normal operating combustion chamber temperature that it is formulated for. This will cause the gas to burn faster. When trying to restart a high compression overheated engine on low octane gas, what is happening is the gas is burning quickly and entirely in the combustion chamber, and producing expanding heat before the piston reaches TDC, driving the piston back down in the cylinder before it reaches TDC. This is the cause of detonation (pounding of the piston) and it'll cause the engine to "grunt" or momentarily make the crankshaft rotate in the opposite direction (but the weight of the flywheel prevents this from happening). Overheating could also crack the [cast iron] cylinder, shrink the piston, burn a hole in the piston (detonation) and weaken the expansion of the piston rings. Methanol fuel has an octane rating of 135. This is why methanol works best in a high-performance, high compression engine.

Did you know that high octane gas or "race car gas" (approximately 109-110 octane) is much better than any filling station pump gas when used in a high performance engine? Higher octane gas burns slow, producing more heat, thus allowing an engine to develop more power. Because heat is how an engine produces power. Methanol fuel produces even more heat than gas does.

Methanol never gets stale while setting around. It's burning factors (the qualities that produce power) remain intact until all of the fuel evaporates. Gasoline on the other hand, as it gets old or stale, it's burning factors is the first thing to go. Unlike methanol, eventually all that is left of the old gas is some "foul smelling residue." Methanol never smells "old."

If the air/fuel ratio isn't properly adjusted with both methanol or gasoline, possible engine damage may result. If either fuel is leaned-out too much, possible piston to cylinder wall scuffing, scoring or even seizure may result. If methanol is ran too rich, the oil on the cylinder wall may be diluted, causing excessive wear to the piston, rings and sometimes the cylinder wall. And if gas is ran too rich, loss in power may result with possible spark plug fouling. (Methanol is more forgiving to an engine than gas.)

A high performance cam, larger piston, big valves and a bigger carburetor will undoubtedly help gas-burning engines develop more power. But only from the use of the parts themselves. Methanol on the other hand, will help give an engine a little more boost. Simply from the fuel itself.

Engines will consume less fuel burning gas than methanol. Something good for gas and not for methanol. Which is why methanol is not used on the open market in daily driven automobiles.

Some racing gasolines could eventually cause an engine to wear prematurely when used under high performance conditions (and if the ignition timing is over-advanced). Piston wear, piston ring expansion failure, valve warping, even motor oil breakdown could result over time due to the high operating temperatures of gas in a high compression engine. (Especially on air cooled engines with no cooling system.)

An engine runs cooler burning methanol than gas. The reason why? Approximately twice the amount of methanol fuel enters an engine. There's more fuel per parts of air. As a result, this allows an engine to operate at a much cooler temperature. Gasoline on the other hand, runs less fuel per parts of air. Therefore, it's a "lean" burning fuel. Gas allows a high-compression engine to operate at a higher temperature. Which in turn could cause premature engine wear. Simply because too much heat itself will cause a good engine to wear out faster than anything else.

For more information on methanol, click here: http://personal.riverusers.com/~yawpower/cfsmeth.html, HHMI Lab Safety: LCSS: METHANOL, Erowid Alcohol Vault : Methanol Material Safety Data Sheet and Race fuel - Alcohol fuels.

Using a Gravity Feed Fuel Flow System -

When burning gas only (not methanol) and if an engine is going to turn no faster than approximately 4,000 rpm (governed limit; the factory setting of maximum rpms for virtually all small gas engines, including all of Kohler engines is 3,600), then a gravity feed fuel system with the gas tank positioned higher than the carburetor, stock fuel fittings and fuel line should work fine. (Gravity feed is defined as when the lowest part of the gas tank is positioned higher than the carburetor, and there's no fuel pump involved.)

If the gas tank sets higher than the carburetor, and if a large i.d. fuel line and fittings are used, and if the float level is set a little high, then a gravity feed fuel system should work great for a stock engine turning no more than 4,000 rpms. But a vacuum-operated (or pulse-type), mechanical or electric fuel pump will guarantee that the carburetor won't starve for fuel about halfway down the track. Also, carefully enlarge the fuel inlet valve seat hole to 7/64" (.109"). This will allow more delivery of fuel in the float bowl so the engine won't starve for fuel halfway down the track. Be sure to set the float a little high too, so as it drops down, the fuel inlet valve will open more, allowing more fuel to be deposited in the float bowl.

FYI - All vacuum-operated fuel pumps operate off of the crankcase pressure. The moving piston(s) inside the crankcase create the pulsating effect to activate the pump's diaphragm. And all crankcase-pulse, vacuum-operated fuel pumps come with three fittings - one that connects to the crankcase of the engine block and an IN and OUT fuel line fittings. It should say on the body of the pump where each fitting connects to. But if it doesn't, then the fitting coming from the center of the pump connects to the crankcase vent. And the other two fittings, one connect to the gas tank and the other connects to the carburetor.

To test a plastic crankcase-pulse, vacuum-operated fuel pump, first check that the fuel line (hose) between the gas tank and fuel pump is open and unobstructed, or if it has dry-rot cracks, which can cause a vacuum leak. To test it, disconnect the fuel line from the fuel pump and wrap a shop towel or clean rag around an air blower nozzle and then insert the nozzle with the rag or towel in the gas tank filler neck. Apply light air pressure in the tank. If fuel comes out of the fuel line, then it's open. After that, reconnect the fuel line to the fuel pump. To test the fuel pump itself, first check the hose that connects the fuel pump to the engine crankcase to see if it has a kink in it or if it has dry-rot cracks, which can also cause a vacuum leak. If it's in good condition, then the final step is to disconnect the fuel line from the carburetor and with adequate amount of fuel in the gas tank, crank the engine and at the same time observe how much fuel is being pumped out of the pump. If there seems to be adequate amount of fuel coming from it, hold your finger over the end of the line and while still cranking the engine, it should build up a small amount of pressure. If none or very little fuel comes out of the pump, then it's bad and needs replacing.

Using a Mechanical Fuel Pump -

Kohler`s Camshaft-Driven Mechanical Fuel PumpWhen turning an engine at wide open throttle, sometimes an engine will starve for fuel with gravity feed fuel system. Therefore, Kohler's camshaft-driven mechanical fuel pump or a low-pressure electric fuel pump should be used to guarantee that the carburetor will receive plenty of fuel at all times. The mechanical pump should be used on a pulling tractor when burning gas only. They're not capable of delivering enough fuel for E-85 or methanol like an electric pump will.

If a Kohler fuel pump produces lower than what seems to be normal pressure or fuel flow, the pump halves are probably warped, causing a vacuum leak. To fix this, separate the pump halves, rotate the diaphragm 90° to remove it from the lever and then resurface the diaphragm flanges on a flat sanding disc. Be sure to thoroughly clean all parts before reassembly. If this doesn't fix the problem, then perhaps a new or good used pump is needed.

To increase the pressure of the mechanical pump (to about 2-1/2 psi), disassemble it and physically stretch the diaphragm pressure spring until it's about 3/4" longer. To increase the volume (fuel flow), the lobe on the camshaft will need to be ground approximately 1/8" deeper.

While the pump is apart, be sure to resurface the pump cover and body diaphragm mating surfaces on a flat sanding disc to prevent any fuel or vacuum leaks. And fuel pump kits are no longer available for any of Kohler's mechanical fuel pumps. By the way - the diaphragm in Kohler fuel pumps is made of very durable material and they rarely go bad. About the only parts that goes bad are the poppet valves, or if water has been sitting in a fuel pump, the aluminum pump cover may corrode and the valves may corrode or become rusted, rendering the pump totally useless.

Mechanical plastic fuel pumps are very durable and they work pretty well, for both yard machines and competitive pulling. The only problem I had with them is when a gasket is used, the plastic mounting flange would warp where it fastens to the block, causing an oil leak. To fix that, I disassemble the pump and use a sanding disc to resurface the gasket area until it's perfectly flat. Then to keep the flange from warping again, I fasten the pump directly to the block using only clear RTV silicone adhesive/sealant with no gasket. And then I install a flat washer under each bolt head to distribute the mounting pressure evenly. And no more oil leak!

By the way - All of Kohler's 10-16hp flatheads and the OHV 18hp engines come with a lobe on the cam to activate the mechanical fuel pump. The fuel pump installs with the lever positioned upwards. And if there's no 3/4" center hole and two 1/4-20 threaded bolt holes on the side of the block to install a mechanical fuel pump, these will need to be drilled and the mounting holes tapped for the engine to accept a fuel pump. Use a fuel pump mounting gasket as a templet.

Troubleshooting a faulty vacuum-operated or mechanical fuel pump -

If an engine sits for about one week or longer, if the fuel in the carburetor evaporates and it's hard to start, or fuel must be squirted into the carburetor to get the engine started, then this means that the vacuum-operated or mechanical fuel pump may be bad. The inlet valve in the pump is leaking, which is allowing the fuel to drain back into the gas tank with the engine off. As the pump tries to syphon fuel from the tank to refill the carburetor, it's unable because the inlet valve is bad. Solution: a new fuel pump will need to be installed.

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New mechanical plastic body "new style" fuel pump for Kohler flathead twin cylinder KT17, KT19, and M18, M20 Magnum series engines (replaces Kohler part # 52-559-01S, 52-559-02 & 52-559-03.) $45.00 each, plus shipping.

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Fuel pump mounting gasket for 10hp-16hp K-series and Magnum flathead engines. Replaces Kohler part # 25-041-09, 25-041-06, 47-041-04 & 52-041-17. .80¢ each, plus shipping.
In-Line Fuel Filter. Replaces B&S 394358S, 4112, SIMPLICITY 173206, 2173206, SUBARU/ROBIN 263-65011-A0. Diameter: 1-1/4", Micron: 75, Fits 1/4" ID fuel line. For engines with gravity feed or fuel pumps. Made in USA. $3.00 each. Part # 20-10347
New in-line fuel shut-off valve. Fits all 1/4" fuel line hoses. Has no filter. Replaces B&S 494768 & 698183. $4.00 each, plus shipping. Part # 23-5841 New elbow shut-off valve with fine mesh screen fuel filter. Has 1/8" NPT taper pipe mounting threads and 1/4" barbed fuel line fitting. Replacement for Gravely, Kohler, Lawn-Boy, Snapper, Tecumseh, etc. Threads into gas tank. $5.50 each, plus shipping. Part # 23-1347
New Fuel Shut-Off/Filter/Sediment Bowl Assembly. Replacement for many older lawn & garden engines. Threads into bottom of gas tank. Dimensions: Overall height: 3"; Maximum width (not including shut-off valve): 1-3/4"; Thread fittings: inlet (top) and outlet (side) are both 1/8" NPT. $10.00 each, plus shipping.
New replacement glass bowl for fuel filter/sediment bowl assembly above. Dimensions: Maximum width: 1-1/2"; height: 1-1/8". $3.50 each, plus shipping.
New bowl gasket for fuel filter/sediment bowl assembly above. Made of gas resistant neoprene. Dimensions: 1-1/2" o.d. x 1-3/16" i.d. x approximately .050" thickness. $1.00 each, plus shipping.

When to Use an Electric Fuel Pump -

When burning E-85 or methanol fuels, more fuel volume (increase in flow) is required and an electric fuel pump should be used. The carburetor on most garden tractor engines don't require a lot of fuel pressure.

Low Pressure Electric/Electronic Fuel PumpsAn electric fuel pump that requires no fuel regulator and works great on virtually any garden tractor engine or up to a 4 cylinder automotive engine with a carburetor is the Facet-Purolator Automotive Electronic Fuel Pump. Facet's part number is FEP 42SV, and Purolator's part numbers are PRO 42S (1.5 - 4 p.s.i.) or PRO 42SV (2 - 3.5 p.s.i.). There's also a Mr. Gasket 42S low pressure universal electric fuel pump, and another electric fuel pump is available at NAPA auto parts stores. It's part # 6101051 (which is actually a FACET # 610-1051 electric pump), and is called a POSI-FLO fuel pump. Specifications are: 12 Volts; 1.5-4 PSI; 25 gal./hr.; compatible with all fuel additives; eliminates vapor lock; two wire design and self-priming. When searching the NAPA auto parts web site, use part # BK 6101051. Low pressure electric fuel pumps are also available on eBay.

The Facet-Purolator pumps are compact, vibration-proof (has solid state dependability), self-priming, easy to install and connect the wiring. They work with either gas, E-85 or methanol fuels, won't flood the carburetor, draw very little amperage from the battery, and they cost less than most OEM electric fuel pumps. They're available at most auto parts stores and on eBay. They're perfect for garden tractors having either a single- or two-cylinder engine!

An electric fuel pump that's made for an import car/truck produce more flow than any of Kohler's mechanical pumps, and in most cases, they don't produce not enough pressure to flood the carburetor. With some high-pressure electric fuel pumps however, an adjustable fuel pressure regulator with a fuel gauge may need to be used in-line. Install a "T" in the fuel line and connect the gauge between the fuel pump or fuel regulator and carburetor. To begin with, set the regulator at about 2-1/2 p.s.i. Make the final adjustment on the fuel pressure at wide open throttle, hooked to the sled, while going down the track. Use a quality fuel pressure gauge that can withstand the vibrations of a garden pulling tractor to monitor the pressure.

Another good thing about an electric fuel pump verses a mechanical pump (if the mechanical pump doesn't have a primer lever, which must be primed manually) is if an engine runs out of fuel, the engine won't have to be cranked for a long time to get the fuel pumped up to the carburetor, which can be hard on an electric starter motor.

How To Prepare a Fuel Pump for Wintertime or Long-Term Storage -

If the engine is equipped with an electric or mechanical fuel pump, apply about a teaspoon-full of clean automatic transmission fluid in the pump so the pump diaphragm will remain flexible or rotor vanes will won't stick and to prevent the poppet valves from sticking or becoming corroded.

DANGER! IMPORTANT! Gasoline and alcohol fuels are both extremely flammable! Use caution and don't smoke, avoid making sparks or don't have an open flame exposed when working near or with either of them. Work in a well-ventilated area, too.

By the way - I've been to a lot of garden tractor pulls over the years and I've noticed that some tractors leak fuel in places where a fuel leak shouldn't be. Therefore, I think that safety inspections needs to include checking the fuel lines for dry rotting and cracks, fuel leaks, loose fittings, etc. The inspectors need to grab hold of the fuel line with their hand and wiggle it around and slightly jerk on it to check for durability of the line. If pullers would build their tractors right to begin with and maintain them, there wouldn't be any fires, or fear of a fire.

Intake tube, carburetor standoff or carburetor extension When choosing the length of an intake tube, carburetor standoff or carburetor extension, it seems that a 3" length works best for gas, and 5" length works best for methanol. Because gas is more lean than methanol, it requires less space or distance to fully atomize, and methanol runs richer, it needs more space or distance to fully atomize before it enters the combustion chamber.

Sometimes when using a long (approximately 5"-6" in length) carburetor extension, and when you modified a carburetor, you really shouldn't need to do anything to the idle circuit. Just make a simple adjustment with the adjusting screw. Therefore, your engine should idle fine. If your engine runs rich at idle, then what is happening is if you're running a cam with a lot of duration and a long carburetor extension, and you're allowing your engine to idle for length of time. What this does is the long duration of the cam lobes, especially the intake valve, the pressure of the piston blows some of the fuel back into the intake tube when the engine is idling. Because an engine has very low vacuum at idle speeds with a long duration camshaft. And when you rev up your engine, a lot of black smoke will blow out the exhaust. If this is what's happening, this is normal and there's nothing that can be done about it, except don't let the engine idle for some time.

If you need an aluminum carburetor extension, contact Lakota Racing (http://www.lakotaracing.com) or Vogel Manufacturing Company (http://www.vogelmanufacturing.com).

How to convert the 10hp-18hp Carter or Kohler carburetor for high-performance use with 100% gasoline, E-85 or methanol fuels - Return To Previous Paragraph or Page È

  1. 10-18hp Kohler CarburetorFirst of all, acquire an older style "26" or "30" Carter or Kohler carburetor. Do not attempt high-performance modifications on a Walbro carburetor. See the drawing to the right for identification of the correct type of carburetor to be modified. è
  2. Before doing anything to the carburetor, take the time to provide a neat and absolutely clean work environment. Make sure that your tools, shop towels and hands are all clean. Don't allow any dust or dirt to enter the work bench or table, including the carburetor and it's internal parts. If necessary, place the parts on a large, clean cloth or cardboard to keep them clean and organized until they're ready to be re-installed in the carburetor body. The best way to clean any carburetor is first remove all parts and then soak the carburetor body and metal parts in Berryman Chem Dip® Professional Carburetor and Parts Cleaner (available at most auto parts supply stores). Don't soak rubber parts in carburetor cleaner because it will deteriorate them. After soaking for about 20 minutes, clean everything with solvent and then use compressed air to clear all the fuel passageways, jets, etc.
  3. Completely disassemble the carburetor. Remove all components from the carburetor including the throttle shaft. Removal of the choke shaft is optional. NOTE: Sometimes the tiny throttle and choke plate screws will break off upon removal because the threads are flared on the backside to prevent them from loosening when the engine is running. So try to be careful not to break them off upon removal. But if they do break off, the broken studs can be drilled out with a 5/64" drill bit, and threads retapped with either a 3-48 NC or 4-40 NC tap so the new screws with split lock washers will thread in with no problems. I know about all of this from years of experience. Or, if you want, I can completely rebuild your carburetor for about $60.00 parts & labor plus return shipping.
  4. Bore out the venturi so the throttle bore will be 1" (.995" NQS legal) or have a straight-through design, if desired. This is a much lower cost but proven performance method than using the aftermarket "Super Carb." Total or partial removal of the venturi is the same as creating a bigger carburetor. A bigger carburetor will allow an engine to draw in more air and fuel, creating more compression and higher rpm, resulting in more power and torque. Total or partial removal of the venturi can be performed with the carburetor securely clamped on the table of a vertical milling machine and then drilled into at a very slow speed using the same size drill bit or reamer that matches the diameter of the throttle bore. Removal of the venturi can also be done on a connecting rod reaming machine. Or very carefully with a handheld pneumatic die grinder. Whatever method is used, just be careful so the carburetor doesn't get damaged and make sure that the throttle bore is straight and smooth, and doesn't exceed the legal limit. But if your club's rules require that the venturi be a specific size, bore it only to that size! And boring of the venturi in the carburetor alone won't help to increase engine performance. To increase the overall airflow in and out of the combustion chamber, and if sanctioning club's rules allow this, the intake (and exhaust) port runners will need to be enlarged, too. This is called "porting an engine." The [stock size] valves will also need to be reworked for improved airflow. Or larger valves will need to be installed.
    IMPORTANT! If you're planning to use your tractor during the winter months for pushing snow or whatever, and because cold air is so dense during the winter (it's actually "thicker" or "more of it"), an engine with a straight-through throttle bore could build up too much compression at full governed speed, and if the cylinder isn't strapped to the crankcase, the high compression could literally blow or break the cylinder from the crankcase or cause an "engine explosion." I've had this personally happen to one of my own engines once and seen it happen to other pulling engines. It's very costly when it happens and not a pretty sight. So unless the cylinder is securely strapped or fastened to the crankcase, it's not a good idea to use a bored-out venturi for wintertime use.
  5. Grind or machine flat the rounded side of the throttle shaft opposite where the throttle plate attaches to so more air will pass around the throttle shaft. The shaft is 1/8" thick in this area. Make it 1/16" thick.
  6. A stock and bored out venturiGrind an angle on the main nozzle:
    • For engines with a stock (OEM) size (1.375") intake valve, grind a 45º-60º angle on the main nozzle toward the throttle plate end. NOTE: An engine with stock valves having a bored out carburetor will accelerate with no problems only if the angle is ground toward the throttle plate. This allows more vacuum (or high velocity of incoming air) upon acceleration to draw the fuel from the main nozzle.
    • For engines with an oversize [aftermarket] intake valve (1.5" or larger), grind a 45º-60º angle on the main nozzle toward the choke end.
    • IMPORTANT: When grinding the angle, be careful not to grind the end of the nozzle too short! It must extend as high as possible into the throttle bore so the incoming rush of air can "grab the fuel" and accelerate the engine without hesitation. Carefully grind 2/3 away (with a bench grinder), but leave 1/3 flat. And be sure to remove the burr after grinding.
  7. Plug the float bowl fuel vapor vent hole with a 1/4" Allen set screw and plug the bottom hole with an 1/8" metal rod.If the engine is going to operate at wide open throttle for pulling competition, the float bowl atmospheric vent hole will need to be plugged with a 1/4" Allen set screw and if there's a small hole in the upper right corner next to the venturi, it will need to be plugged also. (See drawing to the right.) The reason these holes should be plugged is because fuel from the float bowl can splash up and travel through the holes and be drawn into the throttle bore, causing the engine to momentarily surge or run rich. After plugging the holes, drill a 1/16" hole into the float bowl vent passageway on the outside of the carburetor body. (See drawing below.) NOTE: The atmospheric vent allows the incoming fuel to push air out of the float bowl and it allows equal air pressure in the float bowl with the atmosphere so an even amount of fuel can be deposited into the bowl. If the hole wasn't there, or if it's plugged and no other atmospheric vent hole is made, fuel will be forced out the main nozzle and the carburetor will flood at all times. And by the way- if you're worried about dust particles entering the float bowl through the drilled outside hole, well, as fuel enters the float bowl, it pushes the air out through the atmospheric vent hole. That's why the vent hole is there. It doesn't draw any air in.
  8. Do not plug the lower hole below the choke plate. It's supposed to be open so the engine can accelerate.
  9. Exploded view of how to modify a 10-18hp Kohler Carburetor[Information for using E-85 or methanol fuels only]
    • For E-85: Carefully enlarge the fuel inlet valve seat hole (the hole where the float valve/needle sits into) and bottom hole in the main nozzle/jet with a 5/64" (.078") drill bit. Each hole must be the same size for equal fuel delivery.
    • For methanol: Carefully enlarge the fuel inlet valve seat hole (the hole where the float valve/needle sits into) and bottom hole in the main nozzle/jet with a #37 (.104"), #36 (.1065") or a 7/64" (.109") drill bit. Each hole must be the same size for equal fuel delivery.
    • NOTE: Enlarging the fuel inlet seat hole will allow more delivery of fuel into the float bowl so the engine won't momentarily die out when starving for more fuel halfway down the track. And do not enlarge any holes when burning gas fuels!
  10. Micro-Chuck. For use with #60-80 gauge wire drills. Grips drills firmly and accurately in various sizes; as small as a human hair. They can be used hand-held, hand-held drills or in a drill press.Enlarge the lower hole on the high speed needle adjuster with a 3/64" (.047") or a #56 through #60 (.0465" through .040") drill bit. This is performed by use of a micro-chuck. Enlarging this hole will create a stronger suction of fuel at idle speed, allowing an engine to draw more fuel through the idle circuit so it will idle well and accelerate better. It will also allow for quicker starts when the engine is warm.
  11. Before reassembling the carburetor, and to prevent metal cuttings from causing any problems, use compressed air to to clear the jets and all the fuel passages in the carburetor body. Try to keep everything absolutely clean!
  12. After the carburetor body and all its parts have been thoroughly cleaned and reworked, reassemble the carburetor using a new carburetor kit and with the float installed in the carburetor, set the float with the carburetor turned upside-down LEVEL or HORIZONTAL with the carburetor body for general yard use and UPWARD for pulling competition, with gas, E-85 or methanol. Setting the float high will allow the float valve to open further off the seat, allowing more fuel to enter the float bowl as engine rpm increase. This too will allow more delivery of fuel into the float bowl so the engine won't momentarily die out when starving for more fuel halfway down the track. See the animated drawing to the right.
  13. Names of Different Parts of Carburetor KitIf your club's rules don't require a working choke, then leave it out and plug the shaft holes with clear RTV silicone adhesive/sealant. And you may have to hand-choke the carburetor or use a remote fuel primer (read about this further down) to get the engine started when cold. NOTE: With the venturi intact in a stock carburetor, removing the choke plate and shaft does not help in the increase of airflow through the carburetor. Simply because if you were to measure the diameter of the venturi, and then measure the opening of the choke area, the choke area is much larger than the venturi, even with the choke plate and shaft installed.
  14. Set the initial idle fuel mixture screw about 2 turns out (more or less, same for gas, E-85 and methanol). Proper high speed main jet adjustment must be made as the engine is at governed speed or full throttle. To make the correct air/fuel mixture adjustments, install the carburetor on your engine, and first make the necessary idle adjustments until the engine idles at a slow and steady speed. Then rev the engine about half throttle and rotate the main adjuster until the engine runs smooth and steady.
  15. That's it! There's no fancy work involved and it doesn't cost [hardly] anything. And the OEM parts can still be used inside the carburetor. Also, if your carburetor came with a coiled spring that rested in the float bowl, leave it out! All it does is apply upward pressure on the float and sometimes will cause the float valve to stick in the closed position. Also, make sure that the intake and exhaust valve clearances (especially the intake valve) is set at a minimum of .010". This will provide adequate vacuum for the piston to draw fuel from the main nozzle at idle speed.

Modifying an ordinary Carter or Kohler carburetor as described above will improve the performance of a pulling engine dramatically, and if performed correctly, it should match the performance of the popular aftermarket "Super Carb." Basically, the Super Carb is a Kohler carburetor that's had the throttle bore enlarged, a larger throttle plate installed, but still have the legal 1.2" venturi.

And remember this: with a reworked (bored-out) carburetor, to keep the engine from "fluttering" and possibly dying out when pulling the sled, just before takeoff with the sled, first, rev up the engine, then slowly ease out on the clutch. Because the engine needs a lot of airflow at high rpm due to the straight-through throttle bore design. In other words, with a bored-out carburetor, if the engine revs at half-throttle upon takeoff, the engine may not be able to reach its full potential or be able to rev up 100% while under a load.

NOTE: If a carburetor have been converted to E-85 or methanol, and to convert it back to gas, either install a stock main jet or solder up the hole in the main jet with ordinary electrical solder and then drill it with a 5/64" (.078") drill bit, which is the stock factory hole size for 100% gasoline. For best results for drilling, use a small metal lathe with the nozzle fastened in a self-centering 3-jaw chuck. After drilling the hole, gently clamp the nozzle in a bench vise with two blocks of soft wood. Then use a hacksaw blade to remove the solder from the screwdriver slot.

Why Flames Come Out the Header Pipe at High Rpms -

Usually in a high performance engine, on a hot summer night and/or at high attitudes, flames will come out the header pipe at high rpms. This is because all the fuel is not being burned in the combustion chamber and some of the raw fuel exits through exhaust valve and the engine heat burns the fuel on its way out. The engine will lose power because of this, too. To try to burn all the fuel in the combustion chamber, try leaning out the air/fuel mixture and/or advance the ignition timing a few degrees.

16hp and Twin Cylinder Air Cleaner AssemblyA reworked carburetor with a bored-out venturi will undoubtedly allow an engine to draw a lot more air at high rpm. The shorter 10, 12 or 14hp air cleaner assembly is too restrictive for this purpose. Therefore, if rules require that you use an air filter, stack two air filters on top of each other (install a longer mounting stud), use an OEM 16hp or twin cylinder air cleaner assembly (they're taller than the 10, 12 or 14hp air cleaners) or install a velocity stack and use an aftermarket K&N air filter, like the ones that Lakota Racing (http://www.lakotaracing.com) or Vogel Manufacturing Company (http://www.vogelmanufacturing.com) sell.

If you pull on a lot of dusty tracks, then it would be worth investing in an air filter to save wear on your engine.

Advertisement: (Updated 12/4/06)
If you need any of the items listed below, please contact me, Brian Miller, at A-1 Miller's Small Engine & Specialty Shop (1501 West Old Plank Rd., Columbia, Missouri. 65203 | Phone: 1-573-875-4033).Please call any day between 12 noon and 8:00 p.m. Central time, and please be patient because I stutter. Fax: 1-573-449-7347. You can also contact me through Yahoo! Messenger: E-mail: pullingtractor@aol.com. All parts listed are NEW, not used.
Carburetor overhaul kit for the Carter and Kohler carburetors that's used on Kohler K-series and early Magnum single cylinder 4hp-16hp, 18hp OHV and twin cylinder cast iron flathead engines. This kit fits all Carter and Kohler #26, #28 and #30 carburetors. Replaces Kohler part numbers: 25-757-01, 275776, 220701 275778, 231555, Includes float fuel inlet valve and seat assembly, fuel baffle splash shield (goes between rubber O ring and float bowl) and bowl seal (O ring). Made of gas resistant neoprene. Made in USA. Original OEM Kohler Parts. $11.50 each, plus shipping. Individual parts listed below. Carburetor overhaul kit for Walbro carburetors that's used on late Kohler Magnum flathead cast iron block single cylinder models M10-M16 and twin cylinder engines. Replaces Kohler part # 25 757 11-S, 25 757 11, 25 757 11-S. Made in USA. $14.00 each, plus shipping.
New float fuel inlet valve and seat assembly kit. Fits all Carter and Kohler #26, #28 and #30 carburetors used on the Kohler K-series and early Magnum single cylinder 4hp-16hp, 18hp OHV and twin cylinder cast iron flathead engines. OEM Kohler part #231555, 200443, 231555, 25 757 01. Made in USA. Included in overhaul kit above. $9.00 each kit. Part # 525-485

Metal-tipped (all metal) float valve only. Use with OEM Kohler brass seat. These are an OEM Kohler part, and no longer available from Kohler. Works great with methanol! $4.00 each, plus shipping. (When available.)
New bowl seal (O ring). Fits all Carter, Kohler #26, #28 and #30 and will stretch enough to fit the Walbro carburetors that's used on the Kohler K-series and Magnum single cylinder 4hp-16hp, 18hp OHV and twin cylinder cast iron flathead engines. Also fits Tecumseh engines with Carter or Walbro carburetors (large bowl). Made of gas resistant neoprene. 2-3/8" diameter. Included in overhaul kits above. $1.00 each, plus shipping. New fuel baffle splash shield. (Goes between bowl seal and float bowl; prevents splashing gas from entering into throttle bore through the atmospheric vent hole.) Fits all Carter, Kohler #26, #28 and #30 carburetors that's used on the Kohler K-series and Magnum single cylinder 4hp-16hp, 18hp OHV and twin cylinder cast iron flathead engines. Made of gas resistant neoprene. Included in overhaul kit above. $2.00 each, plus shipping.
New float bowl for Briggs & Stratton 7hp-12hp vertical shaft engines. Also fits perfectly on all #26, #28 and #30 Carter and Kohler carburetors that's used on the 4hp-16hp Kohler K-series and early Magnum single cylinder flatheads, twin cylinder flatheads and 18hp OHV (K361) single cylinder cast iron engines. Made of corrosion-resistant zinc-plated steel, inside and out. Dimensions: 2-21/64" o.d. x 1-17/64" height w/3/8" bottom hole. $4.50 each, plus shipping. NOTE: These zinc-plated steel float bowls are better quality than the aluminum Kohler float bowls because they won't rust through, dent or corrode. These are identical in size and dimensions to the OEM Kohler float bowl, too. And the float bowl for the 10-16hp carburetors will also fit the 7 and 8hp carburetors. The smaller retaining bolt will seal well enough against the 3/8" hole and it won't leak fuel. Bowl Drain KitBowl Drain Assembly. Replaces Tecumseh # 27136A & Walbro # 154-503. But can be installed on Kohler and other float bowls to easily drain the fuel after a pull or for winter storage. Eliminates loosening the retaining bolt to drain the fuel. $3.00 each, plus shipping.
New bowl retaining screw gasket (soft aluminum washer) for #26, #28 and #30 Carter, Kohler carburetors with 5/16" or 3/8" short bolt. Please specify diameter of screw. $1.00 each, plus shipping.
New brass float. Fits all Carter and Kohler #26, #28 and #30 and early Walbro carburetors that's used on the Kohler K-series and early Magnum 4hp-16hp, 18hp OHV single- and twin-cylinder cast iron flathead engines. Dimensions: 15/16" i.d. x 1-15/16" o.d. x 5/8" tall. $6.00 each, plus shipping. New plastic float for Walbro WHG and WHL carburetors that's used on all late Kohler Magnum cast iron flathead engines. Replaces (OEM): Kohler #25 757 09, 25 757 09-S and Walbro #75-566, 75-566-1. Dimensions: 2-1/16" o.d. x 25/32" height x 3/4" x 1-1/16" center opening. Made in USA. $8.00 each, plus shipping.
New brass main nozzle. Fits all Carter and Kohler #26 and #30 carburetors used on the Kohler K-series and early Magnum single cylinder 10hp-16hp flathead engines, 18hp OHV and twin cylinder cast iron flathead engines. This particular part is not available from Kohler. Professionally CNC machined in the USA. Drilled for gas. $15.00 each, plus shipping. NOTE: When rebuilding a plain, stock Carter or Kohler carburetor, there is no need to remove the main nozzle! If the main nozzle won't come out, then don't worry about it. All you have to do is make sure the bottom hole is open. But if you're modifying a carburetor to bore the venturi, then the main nozzle will need to be removed. And unfortunately, sometimes they get stuck in place and the screwdriver slot strips out. (I hate when this happens.) I found the only way to remove them is by drilling them out. Because soaking it with penetrating oil or using a small blunt punch to rotate it counterclockwise does not help at all.
New adjustable high speed main fuel brass needle valve. Fits all Carter and Kohler #26, #28 and #30 carburetors used on the Kohler K-series and early Magnum single cylinder 10hp-16hp flathead engines, 18hp OHV and twin cylinder cast iron flathead engines. Replaces Kohler part # 235148, 235415. Professionally CNC machined in the USA. $15.00 each, plus shipping.
  • Carburetor mounting gasket for 4hp, 7hp and 8hp Carter model N carburetor that's used on the Kohler K-series and Magnum cast iron block flathead engines. (1/32" thickness) .60¢ each, plus shipping.
  • Carburetor mounting gasket for all Carter, Kohler and Walbro carburetors used on the Kohler K-series and Magnum single cylinder 10hp-16hp, 18hp OHV and twin cylinder cast iron block flathead engines. (1/32" thickness) .60¢ each, plus shipping.
  • Air cleaner mounting gasket for Carter model N carburetor that's used on the Kohler K-series and Magnum single cylinder 4, 7 and 8hp cast iron block flathead engines. $1.00 each, plus shipping.
  • Air cleaner mounting gasket for Carter, Kohler and Walbro carburetors that's used on the Kohler K-series and Magnum single cylinder 10hp-16hp, 18hp OHV and twin cylinder cast iron block flathead engines. $1.20 each, plus shipping.
IMPORTANT!! A worn throttle shaft is the #1 cause of most engines wearing out prematurely. If a carburetor has a worn throttle shaft, this will create a vacuum leak and the engine will idle poorly, if at all at times. More than .010" of play is considered too much for throttle shaft wear. Plus, at operating running speeds (3,600 rpms), the extra air will cause the engine to run lean on fuel, which will overheat the combustion chamber and cause the cylinder head to warp and the piston and rings to wear prematurely, eventually resulting in severe engine wear and excessive oil burning. Along with regular maintenance, repair of a loose throttle shaft is required to help an engine last a long time. The most accurate way to check for amount of wear is to use a dial indicator.
Throttle Shaft Bushings -
  • Mild steel throttle shaft bushingMILD STEEL bushing for the Carter model N carburetor used on the Kohler 4hp, 7hp and 8hp (models K90/91, K141/K161 and K181) flathead cast iron block engines. Dimensions: either .213" or 7/32" i.d. x .407" o.d. x 1/8" width. $5.00 each, plus shipping. NOTE: Some of these carburetors have a different size throttle shaft. It's either a .213" (#3 number size [drill bit]) or 7/32". Have your shaft accurately measured before ordering so I can drill the right size hole. And making these steel bushings work right can sometimes be a problem. Whenever a bushing binds on a throttle shaft in a carburetor that I have here in my shop for rebuilding, I "fit" or align the bushing with the shaft with either with a drill bit or reamer. Also, not all Carter model N carburetors will accept a bushing. Please check your carburetor before ordering.
  • Bronze throttle shaft bushingBRONZE bushing for the #26, #28 and #30 Carter, Kohler and early Walbro carburetors used on the Kohler 10-16hp flathead and 18hp OHV cast iron block engines. Dimensions: 7/16" o.d. x 1/4" i.d. x 1/8" width. $4.00 each, plus shipping.
  • Installation instructions: The old throttle shaft must be reused with these bushings. The bushing makes contact with the upper unworn part of the shaft. To install the throttle shaft bushing, remove the two tiny screws that secures the throttle plate to the shaft, then lift the shaft out of the carburetor body. Clean out the counterbore and use a 3/16" or 1/4" bolt and small hammer to squarely drive the bushing into the counterbore of the carburetor body and reinstall the shaft. Should one or both of the tiny brass screws break off while being removed, sometimes the shaft can be difficult to remove. To remove the shaft, grind the flared end of the [broken] stud enough where they are flush with the throttle shaft. A Dremel or equivalent grinder with a small chainsaw sharpening stone works best. Then the broken stud must be drilled out of the throttle shaft with either a 5/64", #46 or 2mm drill bit and retap the threads. The stock screw size is 3-48 NC. Should the threaded hole become enlarged from drilling out the broken screw, then a larger screw (4-40 NC) must be installed. In this event please contact me and I will send you the larger screws with split lockwashers.
  • Additional note: Usually, the throttle shaft bushing will snug up a worn throttle shaft. The bushing will last a long time, too. But sometimes the hole in the carburetor body where the throttle shaft fits into will wear also. When this happens, a new shaft won't fix the problem. The hole must be accurately bored and a longer bushing must be installed with the new shaft. But if a new throttle shaft isn't available for your particular carburetor, then one must be machined from scratch. And if necessary, if the carburetor body is worn, it can bored for installation of a longer bushing to snug the new shaft, or a slightly oversized shaft can be installed instead. I can do this for $35.00-$45.00 (depending on type of carburetor), plus return shipping, if interested.
  • And as for the newer style Walbro carburetor without the counterbore for installation of an 1/8" tall bronze bushing, I can accurately bore the carburetor body for installation of a long upper bronze bushing and then if it needs it, I'll install a new throttle shaft. (I've done this before many times with great results.)
    • $50.00 installation of bushing and labor only (without new throttle shaft), plus return shipping.
    • $75.00 with new throttle shaft, installation of bushing and labor, plus return shipping.
Stainless steel throttle and choke plate retaining screws with split lock washers for all Carter, Kohler and Walbro carburetors that's used on the Kohler K-series and early Magnum cast iron flathead engines. Tensile strength: 60,000 psi. Sizes available: 3-48 NC (most common; .096" diameter) and 4-40 NC (.109" diameter) x 1/4" length. Please specify which size you need. .50¢ per pair, plus shipping. Other small screw sizes are also available. Please contact me and I'll see if I can get what you need at a reasonable price. NOTE: When the two tiny brass screws are removed from the throttle shaft for installation of the throttle shaft bushing, one or both of them may break off. If this happens, the remaining protruding threads in the shaft will need to be ground down so the shaft can be removed from the carburetor body. Then the broken screw(s) in the shaft will need to be drilled out (preferably in a drill press) with a 5/64" drill bit and the threads recut so the new screw(s) will thread in with no problems. I know about all of this from years of experience. Or, if you want, I can completely rebuild your carburetor for $60.00 parts & labor plus return shipping.
10-18hp Kohler (or Carter [model N]) CarburetorNew throttle shaft repair kit for Kohler carburetors used on the Kohler K-series and early Magnum cast iron block single cylinder 10hp-16hp flathead and 18hp OHV single cylinder engines. Replaces Kohler 25-757-16, 25-757-16-S and Gravely 214997. Made in USA. Kit includes steel throttle shaft, bronze throttle shaft bushing, throttle shaft bushing installation tool, one #2-56 NC screw, three 3-48 NC screws and Loctite. $35.00 each, plus shipping. Part # 055-329
NOTE: New shaft has no lever - you must reuse the old one. This kit is designed for the Kohler carburetor, like the one on the right, and not the older Carter carburetor having the short throttle shaft with the aluminum clamp-on lever and a flat on one side of the shaft. For the Carter carburetor, use the bronze bushing above.

Repairing a Worn (or Loose) Throttle Shaft -

IMPORTANT!! A worn throttle shaft is the #1 cause of most engines wearing out prematurely. If a carburetor has a worn throttle shaft, this will create a vacuum leak and the engine will idle poorly, if at all at times. More than .010" of play is considered too much for throttle shaft wear. Plus, at operating running speeds (3,600 rpms), the extra air will cause the engine to run lean on fuel, which will overheat the combustion chamber and cause the cylinder head to warp and the piston and rings to wear prematurely, eventually resulting in severe engine wear and excessive oil burning. Along with regular maintenance, repair of a loose throttle shaft is required to help an engine last a long time. The most accurate way to check for amount of wear is to use a dial indicator.

No throttle shaft bushing of any kind is originally installed in the Carter, Kohler or early Walbro carburetors. The throttle shaft rotates directly in the aluminum carburetor body. If the throttle shaft in your carburetor is loose due to many years of normal use, it can be easily repaired by installing a throttle shaft bushing for a snug fit. The bushing will make contact with the upper, unworn portion of the shaft. Doing this will help an engine to idle much better and accelerate with less hesitation. I can fabricate and install a sleeve or bushing to snug up the shaft, or fabricate a new throttle shaft on virtually any carburetor, if you don't mind sending your carburetor to me.

How to Fabricate and Install the Bushing...

  1. Fabricatred throttle shaft bushingThe only tools that's needed to install a new throttle shaft and bushing in a 10hp-16hp flatheads and the 18hp OHV engine Carter, Kohler or early Walbro carburetor are a small flat screwdriver to remove and install the tiny screws in the throttle shaft, a hand or bench grinder to remove the lever from the old shaft and a small hammer to drive the new bushing in the counterbore in that's in the carburetor. The counterbore for the bushing is already made in the Carter, Kohler and early Walbro carburetors.
  2. For the 10hp-16hp flatheads and the 18hp OHV engine Carter, Kohler or early Walbro carburetors, the bushing can be fabricated of either round stock of bronze or mild steel. The dimensions are 1/4" (.250" or .251") i.d. x 7/16" (.4376") o.d. x 1/8" (.125") thickness.
  3. For the 4hp, 7hp and 8hp Carter model N carburetor, the bushing needs to be made exactly .213" i.d. (#3 drill bit) x .407" o.d. x 1/8" (.125") thickness.
  4. Remove the throttle shaft from the carburetor body. Should one or both of the tiny brass screws break off while being removed, sometimes the shaft can be difficult to remove. To remove the shaft, grind the flared end of the [broken] stud enough where they are flush with the throttle shaft. A Dremel or equivalent grinder with a small chainsaw sharpening stone works best. Then the broken stud must be drilled out of the throttle shaft with either a 5/64", #46 or 2mm drill bit and retap the threads. The stock screw size is 3-48 NC. Should the threaded hole become enlarged from drilling out the broken screw, then a larger screw (4-40 NC) must be installed. In this event contact me and I will send you the larger screws with split lockwashers.
  5. Check that the bushing fits on the shaft snugly and rotates freely (by hand). If it fits too tight, the bushing may need to be reamed out slightly to accept the shaft for a free fit.
  6. Create a bevelled edge on the bushing so it will slip into the counterbore easily. The counterbore for the bushing is already made in the Carter, Kohler and early Walbro carburetors.
  7. If necessary, clean the counterbore of any debris and entire carburetor.
  8. Using a hammer, gently drive the bushing (with a short 1/4" bolt in the bushing for protection) squarely into the counterbore until it bottoms out. (See pictures to the right.)
  9. Install the throttle shaft, plate and screws w/lockwashers and that's it!

NOTE: When the two tiny screws are removed from the throttle shaft for installation of the throttle shaft bushing, one or both of them may break off. If this happens, the remaining protruding threads in the shaft will need to be grind down so the shaft can be removed from the carburetor body. Then the broken screw(s) in the shaft will need to be drilled out (preferably in a drill press) with a 5/64" drill bit and the threads retapped with either a 3-48 NC or 4-40 NC tap so the new screw(s) will thread in with no problems. I know about all this from years of experience. I sell new screws with split lock washers below if you need them. Or, if you want, I can completely rebuild your carburetor for about $60.00 parts and labor plus return shipping. And the carburetor on the Kohler engine model K532 have no counterbore for a throttle shaft bushing. Therefore, either the throttle shaft holes in the carburetor body will need to be reamed out for installation of bushings, or an oversized throttle shaft must be fabricated and installed.

By the way - I sell new bronze throttle shaft bushings. I also sell new Kohler-made throttle shaft repair kits. Click here to check out some of my products. Return To Previous Paragraph or Page

Never use multiple gaskets when mounting the carburetor to the engine block or intake tube! The use of two or more gaskets will severely warp the flange on the carburetor, causing it to bend, which will create a vacuum leak, or the flange could even break. So remember, first resurface the mounting flange and use just one thin (.030" thickness) gasket.

Or being there's no long intake tube for the gas to atomize in before it reaches the combustion chamber, the Kohler OEM thick carburetor mounting gasket (which is actually a piece of plastic-like material sandwiched between two gaskets) isolates the carburetor from the engine heat so the gas will atomize better when it before it enters the combustion chamber. Atomized gas vapors mixes with the air better, and make the engine produce more power.

Got a stuck throttle shaft?

How to get a rusted-in-place throttle shaft loose: Visit a local Dollar General store and buy a spray can of GUNK Liquid Wrench. This stuff works wonders on stuck throttle shafts and all rusted or corroded metal parts! Just spray the top and bottom of the shaft, let soak for about 30 minutes or maybe overnight, then VERY GENTLY, use Vise Grips to rotate the shaft back and forth. Be very gentle. Don't force it loose because it could become twisted and/or break off! After it rotates slightly, spray it again and gently rotate it back and forth again. Eventually, it'll loosen freely.

But if it still refuse to loosen, remove the throttle plate, float bowl, float and baffle shield. Then drill a 3/32" hole in the carburetor body directly in-line with the bottom of the throttle shaft. Finally, use a small roll-pin punch to drive the shaft from the carburetor body. If either of the throttle plate screws breaks off, be sure to grind off the protruding stud that remains for easy removal of the shaft. Cut some short 5-44 NF threads in the drilled hole and then plug the hole with a 5-40 NC Allen set screw (the difference in threads will not allow the set screw to go all the way through the hole and interfere with the throttle shaft) and then grind the protruding end of the set screw so it won't interfere with the operation of the float.

Should one or both of the tiny brass throttle plate retaining screws break off while being removed, sometimes the shaft can be difficult to remove. To remove the shaft, grind the flared end of the [broken] stud enough where they are flush with the throttle shaft. A Dremel or equivalent grinder with a small chainsaw sharpening stone works best. Then the broken stud must be drilled out of the throttle shaft with either a 5/64", #46 or 2mm drill bit and retap the threads. The stock screw size is 3-48 NC. Should the threaded hole become enlarged from drilling out the broken screw, then a larger screw (4-40 NC) must be installed. In this event please contact me and I will send you the larger screws with split lockwashers.

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Modifications for the 10-18hp Kohler Carburetor If you need your Carter, Kohler or Walbro carburetor rebuilt for daily use or if you want your carburetor professionally reworked for high-performance use on a Stock, Stock-Altered or Pro-Stock tractor, or if you wish to buy a rebuilt carburetor, please contact me, Brian Miller, at A-1 Miller's Small Engine & Specialty Shop (1501 West Old Plank Rd., Columbia, Missouri. 65203 | Phone: 1-573-875-4033). Please call any day between 12 noon and 8:00 p.m. Central time, and please be patient because I stutter. Fax: 1-573-449-7347. You can also contact me through Yahoo! Messenger: E-mail: pullingtractor@aol.com.
IMPORTANT: When or if sending your carburetor to me, package it securely so it won't get damaged in shipping and please include a note in the box with your name, complete mailing address, phone number (in case I have any questions) and a description of what you want done. And be sure to enclose it in a sealed zip-lock sandwich bag to prevent the gas odor from escaping the package. If the odor of gas can still be smelled from outside the box, then apply perfume or cologne on the carburetor to disguise the smell of gas. Because the US Postal Service or other delivery services will not deliver any packages that smell like gas. And we'll call or email you when it's repaired.

Carburetor Work:

For a basic rebuild only with no "high performance" modifications whatsoever, the price is $40.00 labor, plus return shipping. Any additional parts or services are an extra charge. Work includes whatever parts and services it may need that's listed below.

Basic Rebuild Includes:

ü Completely disassemble carburetor, soak and clean it in Berryman Chem Dip® Professional Carburetor and Parts Cleaner and if necessary, thoroughly clean it afterwards with bead blaster (if the paint will come off).
ü Resurface mounting flange and choke end.
ü Install a new throttle shaft bushing for a snug fit. (If required, $25.00 extra. There's a $5.00 extra charge per screw for drilling out broken-off throttle plate screws.)
ü Enlarge the lower hole and apply heat shrink wrap tubing over the tiny holes midway on the high speed needle adjuster so the engine will start easier and idle smoother.
ü Install new overhaul kit.

High-Performance Upgrade for Gas, E-85 or Methanol fuels - $100.00 labor, plus return shipping. Work includes whatever parts and services it may need that's listed below.

High-Performance Modifications Includes the Above Work, Plus:

ü Bore the venturi to match the diameter of the throttle bore or 1" limit (.995" NQS legal size) or whatever your club's sanctioning rules allow. Read your rules carefully to make sure! I'll need a copy of your club's sanctioning rules regarding the legality of the carburetor, so when I rework it, it'll be legal for use in the class you're planning to pull in.
ü Grind an angle on the end of the main nozzle. (Only if venturi is bored.) NOTE: If the main nozzle is stuck and must be drilled out to be removed, a new one will be installed. Removal of the stuck nozzle and installation of a new one is $30.00 extra, which includes a new nozzle. So please apply penetrating oil to the main nozzle before you send me your carburetor.
ü Grind a flat place on the throttle shaft opposite the throttle plate so more air can pass over it.
ü For engines that operate above 4,000 rpm, I plug the original float bowl atmospheric vent hole and drill a small hole on the outside.
ü The choke plate and shaft remains intact, and is still usable. Or if you want, I can remove the choke plate and shaft and then plug the shaft holes with silicone sealer. If your rules require a choke or if you wish to have a working choke, please let me know and I will not remove it.
ü Install a new overhaul kit.
ü Install a new throttle shaft bushing (if required, $25.00 extra.)
ü Add $20.00 for converting to bottom main fuel adjuster.
ü Missing, worn or broken parts are extra charge.
Professionally Rebuilt Carburetors For Sale -

Rebuilt to OEM specs, Carter model N carburetors for models K141/K161 (7hp), K181, M8 (8hp) Kohler K-series and Magnum cast iron block, flathead engines. These are plain, stock carburetors that's been completely disassembled, then soaked and cleaned with Berryman Chem Dip® Professional Carburetor and Parts Cleaner and if necessary, thoroughly clean it afterwards with bead blaster (if the paint will come off). All fuel passageways cleared, new parts and a new throttle shaft and/or bushing are installed if necessary. There are no modifications made to these carburetors whatsoever. [When available.]

  • $100.00 each outright, plus $6.00 shipping in the Continental U.S.
  • IMPORTANT: When or if sending your carburetor to me, package it securely so it won't get damaged in shipping and please include a note in the box with your name, complete mailing address, phone number (in case I have any questions) and a description of what you want done. And be sure to enclose it in a sealed zip-lock sandwich bag to prevent the gas odor from escaping the package. If the odor of gas can still be smelled from outside the box, then apply perfume or cologne on the carburetor to disguise the smell of gas. Because the US Postal Service or other delivery services will not deliver any packages that smell like gas. And we'll call or email you when it's repaired.