In another thread I have mentioned my difficulties with an OS91FX(two stroke) using a pressurized fuel system. I was using a Cline Regulator taking crankcase pressure for the tank. The UltraThrust muffler made the engine almost not run. Dave at Performance Specialties told me that the muffler pressure and crankcase pressure cycles interfere so that the engine will not run correctly. I know the YS pumped engine "may" operate somewhat differently than the Cline system, but I don't feel like buying another UltraThrust/Jett Stream type muffler only to find it is also incompatible.