Information 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 four types of carburetors for their K-series single cylinder 10hp through 18hp engines and KT-series opposed twin cylinder engines.
- The first carburetor is a number "26". It's used on the 10hp (model K241), 12hp (model K301) and small opposed flathead twin cylinder engines. It has a 26 millimeter or 1.07" diameter throttle bore. The venturi measures .812" in diameter.
- The next is a rare one, number "28". It's used mostly on the 14hp (model K321) and on some 16hp (model K341) engines. It has a 28 millimeter or 1.17" diameter throttle bore. The venturi measures .937" in diameter.
- The third is a number "30", which is also used on the 14hp, 16hp, 18hp OHV (model K361) and larger opposed flathead twin cylinder engines. It has a 30 millimeter or 1.200" diameter throttle bore. The venturi measures 1.000" in diameter.
- The venturi in the #30 carburetor that's used only on the 18hp OHV single cylinder engine measures .1.062" in diameter.
- By the way - the numbers on the carburetor mounting flange refers to part numbers. Please refer to Kohler publications TP-2379 single cylinder engine manual, and P-2377-D carburetor reference manual.
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. 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". If there's no number embossed, you'll need to accurately measure the diameter of the throttle bore to determine the size carburetor you have. Airflow -
- The venturi in a stock #26 carburetor measures .808" in diameter, and flows 74 CFM. With the venturi removed (bored out venturi, making it have an unrestricted straight-through throttle bore), this carburetor will flow 98 CFM (cubic feet per minute), which is about 1.3 times or 65% more airflow.
- The venturi in a stock #28 carburetor measures .920" in diameter, and flows 84 CFM. With the venturi removed, it'll flow 107 CFM, which is about 1.3 times or 75% more airflow. A bored-out #26 carburetor will out-perform a stock (non-bored) #28 carburetor at high rpms. And the throttle bore in a #26 Carter or Kohler carburetor cannot be bored large enough to match the size of a #28 carburetor. The throttle bore "wall" is simply too thin.
- The venturi in a stock #30 carburetor measures .920" in diameter, and flows 84 CFM. With the venturi removed, it'll flow 110 CFM, which is about 1.3 times or 77% more airflow. A bored-out #26 carburetor will out-perform a stock (non-bored) #30 carburetor at high rpms. And the throttle bore in a #26 Carter or Kohler carburetor cannot be bored large enough to match the size of a #30 carburetor. The throttle bore "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 for improved engine performance, and if sanctioning club's rules allow this, the intake (and exhaust) port runners must be enlarged, too. This is called "porting an engine." The [stock size] valves will also need to be reworked for more airflow. Or larger valves and a bigger cam will need to be installed.
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 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.
If you have a 14hp Kohler engine, and you can't find a #28 or #30 carburetor, then a #26 carburetor will work just fine for ordinary yard use. Personally, we ran a stock #26 on the 14hp engine that's on our 6,000 lb. self-propelled pulling sled for 10 years and the engine didn't lack any power whatsoever.
Identification of Walbro Carburetors -
Walbro 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. We have an assortment of tiny drill bits that we use on our customer's lawn & garden equipment carburetors. 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. And the new style Walbro carburetors are notorious for leaking gas, even after installing a new kit with a new float valve and seat. This is undoubtedly how big business keep selling new carburetors nowadays. Anyway, most newer Walbro carburetors aren't worth rebuilding. If it can be done, install a rebuilt Carter or Kohler carburetor instead. If it's on an engine that requires a difference bolt pattern for mounting the carburetor, an adapter may have to be made. But the older Walbros are worth rebuilding, because they're much like the Carter and Kohler carburetors. They were made back in the day when most businesses took pride in their products.
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 the metal is simply too thin for welding or a Heli-Coil insert.