I had a side mont mufler on it with 2huge outlets on it for several years...After I put the pitts on it, I had a big powerloss and had to richen the needles a lot. I removed the outlets to hopefully let the engine breath a little better but it apparantly didn't help.

I just put it all back together and ran it... I set the timing back just a hair and didn't hear the noise any more...I don't have the cowl on the plane and the engine was still pretty hot after running about 1/4 tank thru it. But I checked my needles and they are almost 5 turns out each... The engine rpm or sound will not change no matter how far I back the high one out, the lowend makes some difference at idle...It's the older bing carb, maybe it's time for a new one, I cant find a kit for the bing.