it only happens at idle. It's perfectly fine at half or even full throttle. I can see whats causing it. AIr gets into the line because the clunk is not sucking the fuel that moves to the foward of the tank. It's not a rich problem, in fact, the idle screw has little to no effect on the engine at idle. Yes it is a great planes cub. ***************I know for sure its a fuel sucking problem, at half tank, the fuel moves to the front of the tank and the clunk at the back cannot pick it up************* What would cause this?