ORIGINAL: usafloadc17
the formers allow no room for the tank to be lowered. Tight fit. And it only occurs at idle, every single time, with different engines. It's a fuel sucking problem. I thought it as the pitts muffler not providing enough back pressure when the nose is down. BTW, hoes the the tank pull fuel from the front when the nose is down? If the clunk stays at the back, back pressure is not enough to feed more fuel to the back is it?
No, lowering the tank has nothing to do with it.
If it did planes would not be able to fly at all of their different attitudes.
The tank pulls fuel because of the lower pressure at the carb coupled with the increased pressure provided by the muffler to the vent line.
As long as the clunk is submerged in fuel, the fuel will continue to flow. That is why a plane can fly nose straight up in the air. The pressure differential causes fuel to move UP to the carb w/o problems.
You say that this only occurs at idle... but you didn't say if the tank is full or almost empty when this occurs.
It would help us if you answered the questions that are asked of you.
So when the plane is at idle with 1/2 a tank or MORE, have you performed a "pinch" test?
e.g. when you pinch the fuel line for a second or so, does the engine speed up slightly?
When you put the nose down about 15 degrees, do you see air getting into the line?
Can the clunk "swing" down a bit as indicated in the above example pictures?
A degree of "swinging" down is very necessary to keep it submerged as the tank empties, and helps forstall the problems I mentioned in my post about clunks becoming uncovered.