If I understand you correctly.....you start in level flight and pull up into the loop (inside loop) with no problems....but inverted and push? into the loop (outside loop) starting at the bottom???????is that correct? the inside loop the fuel will stay in the bottom of the tank...the outside loop fuel will move to the top of the tank....sounds like most probable the clunck is sticking to the back of the tank not allowing free movement in the loop to the top of the tank......or the tank is too big and you don't have enough head pressure in the tank from the muffler......let us know what you find</p>