I think I have the answer to your problem. A lot of guys fill the fuel tank quite a while before they start their engine. Don't fill the fuel tank untill you are ready to start the engine. Then don't choke the engine. If the engine is inverted turning the engine over with the electric starter will prime and start the engine all in one step. If you fill the tank and then let the plane sit for a few minutes or longer the fuel will siphon into the carburetor.