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1 G Carbs

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mstull

R.I.P.
Joined
Jun 23, 2005
Messages
1,263
Location
West Texas
I noticed my engine dies, if I pull a lot of Gs. I hope this thread is helpful and thought provoking for y'all.

With float/slide carbs, common on 2 stroke engines, the fuel mixture becomes leaner the more Gs you pull. With this type of carb, the venturi effect suction has to suck fuel up out of the float bowl 1/2" or so. When you pull Gs, even though the float level and suction are the same, the fuel is heavier. So the suction will pull less fuel up through the jets.

Picture it like sucking water out of a bucket through a hose a couple feet up. It takes a certain amount of suction to get it up that high. Then picture trying to suck a much heavier liquid up that high, like mercury. It would take much more suction to lift the liquid.

This effect is pretty instantaneous. Your engine goes leaner the moment you pull Gs. And how lean it gets is proportional to how many Gs you pull. How many Gs it takes for your engine to stop producing power, depends on how rich your mixture was to begin with.

To test this out, cruise at a certain power setting and note the EGT. Then start doing steep banked turns at the same power setting, and watch the EGT come up.

The leaning is greatest at high throttle settings, where the air velocity through the carbs is least (even though the air volume flowing through the carbs is greatest).

Changing to a higher pressure or volume fuel pump doesn't help at all. The floats will keep the fuel level in the float bowl the same. If you want to be able to pull more Gs without killing your engine, you'll have to tune your carbs richer, throttle back, or apply choke.

A second problem with pulling Gs... If your fuel tank is well below your fuel pump, the pump might not be able to suck the fuel up that far in high G loads as well. The fuel will tend to vaporize at the top of the fuel line near the pump, creating a bubble... especially at high temperatures. This is often called vapor lock.

This problem will stop the flow of fuel into your float bowls. It takes a few seconds for the engine to use up the fuel in the float bowls. So in this situation, the engine won't lean out for a few seconds. Again... this vapor lock problem only occurs if your fuel tank is well below the fuel pump.
 
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