It's not that the engine should work with ANY muffler, but that the muffler makers should make sure their mufflers work with the O.S. engine. O.S. cannot design their engine to someone else's muffler design. Please remember that many muffler makers don't really take the time to work out all the kinks in their mufflers. They basically make a generic "can" and then adapt the fittings so that it will bolt/clamp onto various engines.

With after-market mufflers, you may have to adjust the exhaust outlet to get enough pressure to the fuel tank to give the O.S. Max .91 FX enough fuel pressure to run well.