by Gordon Jennings

"In a go kart you don't change the horizontal plane of the carb or float chamber when cornering. This causes fuel to climb the walls of the float bowl and give a false reading to the float & needle. We run catch cans to catch this fuel that overflows the bowl out the vent.

"I'm not sure if the carb is high enough in relation to the fuel tank to run a return line from that nipple on the bottom of the fuel bowl back to the tank.

"I use a PKW 28mm Kiehn carb on my 80cc Kawasaki with a Stewart Warner fuel pressure reg set @ 1.5 psi and get fuel in the catch can every lap. The amount is according to the track layout and how severe you brake or corner.

"I talked to a guy with a 125c Honda laydown kart that had made up a larger float bowl for his Mikuni and he claimed this is the solution.

"I was thinking of going to a 34mm Tillotson pumper carb from a snowmobile. My carb is at a 10 degree angle-tilted down in front (the engine is mounted with pipe to front and carb to the rear) so I guess any braking would pile the fuel forward big time . Any thoughts?

"I should have told you the rules state that we can run 100cc and I am doing that. A 25% increase in cc's should take care of some of the extra fuel from a 34mm carb.

"Please elaborate on the off idle problem with a pumper carb. Using a pumper and being able to tune as you drive while watching the exhaust gas temp gage seems to be a good idea.

Karts corner (korner?) much, MUCH harder than any of you motorcycle or sports car guys could ever imagine. They're like Formula One cars before aerodynamic downforce.

I'm not surprised that float-type carburetors slosh fuel while the kart is cornering. At at a cornering load of 1.3 gravities, which a kart can achieve, the fuel level in a carburetor's float bowl is tilted about 57 degrees off the horizontal.

In pre-fuel injection days people in American track racing used floatless carburetors. These carburetors had a kind of standpipe surrounding the jets. Fuel was delivered into the standpipe and the excess spilled over its top -- which was set at the height otherwise provided by the floats and float valve -- and dribbled down into the "float" bowl. From there the excess fuel was returned to the fuel tank by gravity or, more often, a second fuel pump.

Rick, who raised this subject, notes that he isn't sure he would get a gravity return to his kart's fuel tank, and he probably is right, which would mean supplying a second fuel pump if he wants to continue with the existing carburetor. But what he needs is a bigger carburetor, and one not so greatly affected by cornering forces. I don't see how a larger float chamber could solve the basic problem, as it would just have more fuel sloshing sideways.

The 34mm Tillotson "pumper" carburetor Rick contemplates using should be a better choice.

Tillotson's snowmobile, chainsaw, etc., carburetors do not have float chambers, and use instead a demand-type flow regulator comprising a diaphragm and needle valve much like the ones that admit fuel to float chambers. It has been Tillotson's practice to include a diaphragm fuel pump -- driven by the rise and fall of crankcase pressure -- in the same stack as the fuel flow regulator. Hence the term, "pumper."

The big problem Rick, or anyone, will face when moving a big carburetor from a big engine to a smaller one is that the carb's fuel metering will have been set for the engine on which it was originally fitted. The 34mm Tillotson would be at the outside limit of size for an 80cc two-stroke engine; it's big for a 100cc engine, but within the realm of good possibilities.

In the Tillotson there are two areas that may need attention:

First, there's the tiny spring that holds the fuel valve shut. The resistance of this spring is overcome when the vacuum on the regulator diaphragm is stronger than it is. When you put the carburetor on a smaller engine, the vacuum signal from the venturi is not as strong, so you may have to fit a weaker spring to bring the metering system into balance.

Second, the small holes upstream and downstream from the throttle butterfly have been sized for the bigger engine and may have to be changed for the smaller one.

At idle, air enters the hole upstream from the throttle valve, joins with fuel from the idle jet, and is then sucked out the hole behind the throttle. As the throttle opens, the throttle valve disk rotates and its leading edge moves forward, quickly applying manifold vacuum over both idle holes, which means the up-stream air bleed hole is made into a downstream fuel hole. The fuel then emerging from both idle holes is supposed to match the extra air coming past the partly-open throttle valve and keep the air/fuel mixture in balance until the main fuel system comes into play.

I can't tell you anything specific about idle system modifications beyond saying that you should start by enlarging the upstream hole VERY slightly. Or, you could drill a second upstream hole of smaller diameter on the downstream side of the first. Both modifications have the effect of increasing the closed-throttle air bleed, meaning you open the idle fuel jet to keep the idle mixture correct. Then as the throttle disk's edge moves over the air bleed holes they become bigger fuel holes and are better able to make the idle-to-main jet transition for the smaller engine.

Rick says, finally, "Watching the exhaust gas temp gage seems to be a good idea."

Almost everyone in karting seems to think ad hoc fiddling with mixture while zooming down the track is a good idea, and they may be right. I don't have enough kart experience to be certain. But when I built a kart for my then-young son, I set the mixture by experimenting a little and "reading" the spark plug. After that, I could have welded the main jet setting in place, because it was never changed again. And my kid had speed on all the McCulloch-engined karts he ran against.

I personally regard exhaust gas temperature (which changes with spark timing as well as mixture) a poor indicator of an engine's fuel requirements. But, then, what the hell do I know about it anyway! Right?