I have a magnum 28 that I am having problems getting fuel to the motor. I put on a new tank, new lines, new clunk and new line from the remote needle to the carb. It will run if I put my finger over the muffler or the carb opening. If I remove the needle valve and blow in the vent line I get all kinds of fuel coming out. This engine to my knowledge and been taken apart, could the piston be in backwards? I am not sure what else to check.