I firest took the baffle out of the muffler on my OS.91FX mounted on a Extra 300, it didn't work too well. Put it back and have switched from 15% to 10% nitro and it works fine. Pulls my 9 lb Extra around pretty good. Very seldom ever have a dead stick. I don't care for the three piece muffler, it's come apart on my a couple times. But overall its been a good engine, bought mine used on E'bay, it has been worth it.