Ditto, inverted shouldn't rob power. About the only issue with inverted engines is they sometimes are hard to start if the plug is soaked with oil. Some fuel may dribble or spray out of the carb when running, this is fairly normal as long as it is not excessive. Likewise, if the carb is below the tank line, you may have some fuel siphon out when the engine is off, also normal. Like mentioned above, I'd look into cooling issues.