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Carburetor Vibration

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mstull

R.I.P.
Joined
Jun 23, 2005
Messages
1,263
Location
West Texas
I figured out why my engine was running rich and dripping fuel out of the float vent. Vibration causes the float needle to vibrate and allow excess fuel into the float chamber. The quantity of fuel, that gets past the needle every vibration, is proportional to the pressure from the fuel pump. The carbs on our engines are designed for motorcycles, with gravity feed from a fuel tank that is just above the carb... with almost no fuel pressure.

I resolved this too-much-pressure problem by putting a tee in the fuel line between the pulse pump and the carb, and running a full size return line from the tee back to the fuel tank. So the pressure to the carb is near zero now. For example... If the pressure is reduced to 1/20th of what it was, you'll get 1/20th as much flow in each vibration. 1/20th is low enough that your engine will consume it fast enough not to overfill the float chamber.

You'll have to completely retune your carb when you're done. But it will tune easily because the fuel level in the float bowl will be where it's supposed to be. And the carburetion will be consistant, relatively unaffected by vibration.

This return line technique should work with any float carb on any vibrating engine, unless your fuel tank is significantly below your carb. In that case, you'll have to use a fuel pressure regulator, or add a headder tank.
 
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