The muffler is not the problem, you probably just have it tuned too rich. I have the tower pipe on my os55ax and it gained 600 rpm and I've gotten 15 minutes flights out of a 8 ounce tank. The pipe is lighter, makes more power, and isn't that much louder than stock. It's a win win win combo. I will buy your tower pipe off you if you're sure you don't want it.

Mike