It is important to make sure that you have the level of the fuel tank at the height recommended by the engine manufacturer. Often times this means that the center of the tank, as viewed from the side, needs to be at the same height as the needle valve of the carburetor. If the tank is too high the gas may siphon out of the tank and the engine may not run well. Once you have the tank mounted correctly then carefully adjust the needle valves for proper operation of the engine. After this is done the engine should run as well as if it were mounted upright.