its a .40 great planes cub. The tank is actually about 2 inches below the need valve which may be a problem, however, the plane is designed like that or the cowl wont fit. It's not an air leak because I bought al new lines and replaced the tank twice. The pitts muffler has 2 gaping holes, maybe its a pressue problem? Whenever I point the nose down, the engine runs for about a minute then dies. Maybe this is normal?