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Leaking carb

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Ok I know what everybody will say it is the needle valve but the carb only drips when the tractor is ideling.If I shut the tractor off it does not drip or if I bring the rpm up no problem.I did rebulid the engine this winter but did not tuch the carb on it (ran good last year with no drips)So any good Ideas?Bob

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Hi, If the intake valve clearance is inadequate or the valve is not seating good can cause this problem. At a slow idle the air flow actually can reverse on the compression stroke if the valve is leaking. At higher speed it doesn't happen because the cycle happens so fast that the voume of leakage is negligible. This is common on most engines and is called spitback. With the new EPA emission levels the manufacturers are putting baffles in the air cleaners etc to address the problem. If the problem is normal it is caused by the valve overlap on the exhaust to intake stroke. The exhaust valve stays open past bottom dead center and the intake valve opens before bottom dead center. When the engine is running at high speed the velocity of the exhaust going through the exhaust valve actually scavenges the cylinder and starts to pull air in the intake valve from this action. At low speeds this action doesn't happen because of the low exhaust velocities. The overlap is expressed in degrees and the more overlap the worse the engine runs at low speed and the more power at high speed. This is why racing engines with "wild Cams" that may actually have the valves open up to 310 crank degrees will barely run below 3000 rpm, but generate tremendous power from 6000 up. Every thing to do with camshafts is a trade off between low speed performance and high speed performanance. Good luck, Al Eden

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