I hope you are referring to the ULTRATHRUST muffler sold by Performance Specialties---'cuz I just ordered one for my O.S. FX.61. The guy over there said you need to use a longer chambered muffler than the one normally made for a .61 if you are going to swing a larger prop than maybe a 12X6---or it won't "needle". That may be the cause of the idle problems you mentioned. I have thus far had best results with a 13X6 APC prop--let's see what this will do for $70---it's only money --right??

BTW-Guy says "...you use it--it's yours...". I'll let you all know.