I also have to dissagree with Eltigre, any of the model engines will run as well upside down as they will right side up. If there are problems in getting a good idle when running inverted, it is just improperly adjusted, most probably on the low speed needle valve or the tank is badly located relative to the carburator. The main caution in starting and inverted engine, especially the two stroke, is getting a fluid lock by overchokeing or by syphoning fuel into the combustion chamber while the plane is sitting unused.