Ok,
First of all this is an engine off a hanger 9 Arrow trainer. It went caput a year ago . so the engine seems fine just a broke needle valve so i ordered one from horizon and installed it on a 40 size twist minus all the trainer stuff that came one this engine. I removed the baffle and got some more horse power but in order to get the tank level with the carb i had to raise it all the way to the top of the fuse. no padding on top of the tank. now here is where the probs come in.The motor runs like a scalded cat when it is in level flight doing circles and figure eights. but roll it on its back it will dead stick going in a down line it will dead stick . I have set the plane on a bench and ran nearly a gallon of fuel throught it trying to figur the cause of the prob. I get a few bubbles in the lines some from the tank some seem to come from the needle valve but most part i think it is caused by vibration due to the tank situation. Now I have installed three different tanks. three different sets of complete fuel lines. Different clunks. different clunk lines .none of which is binding at the back with full freedom of movement. when I test the plane at half throttle and simulate inverted flight just like it would do in the air the engine bogs down and dies. but I see no bubbles when I do this. What is my problem?