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?