Hi, have the same engine, muffler and airplane. This engine with the stock muffler was working perfectly on an other airplane. Had to plug one of the exhaust ports on the pitts muffler to increase exhaust pressure to the fuel tank so it could work ok. If the carb was adjusted to work ok when the plane was level, the engine would died when it was hold in a nose up position, if it was adjusted to work ok nose up, there was nothing to do to have the mid-range work properly when level. A tight piece of wood with a screw through the exhaust pipe works ok. Suggest plugging the left one for better gas evacuation. Also check if the lower front right side of the fuel tank is rubbing against the triangular piece of wood glued for re-inforcing the fuel tank compartment. Mine was and it cut my tank, still waiting for it. Had to call them, send e-mail with pictures for a replacement. They dont sell this tank separately.