I just bought a used saito 90t flat twin engine(twin carb).the problem i am facing is the perfomance of the engine.The engine is mount on a world model P-39 running a 13 x 6 prop (6800 rpm).I still fly it and the plane is not performing (speed) i changed to a 12 x 6 and it does improve abit.so my question is , are the engine underpower for the plane.I am running 15% cool power.I have disassemble the engine and check the timing and adjusted the tappet.Which is the most suitable prop for this engine.Is it true that a 90 twin is less powerful than a 90 single piston.