if the intake on your four stroke is filling up with fuel, then your tank is too high--plain and simple. lower the tank and the engine will run great inverted. In actuality, four strokes tend to do better inverted than two strokes because most two strokes have the glow plug right in the middle of a domed cylinder head--so fuel will run right down to it before it gets vaporized. four stroke plugs are canted, and usually not where fuel can collect on them during the intake and compression strokes.