ZDZ40 what rpms are you guys getting ?
The S&W muffler is probably your problem. I have one but have not tried it yet. They are very quiet but also very restrictive. The Bisson is also restrictive.Take the muffler off and see what kind of numbers you get. I think it will perform the way it should. The best muffler for ZDZ's are a canister type if you have the room, if not try the J&A.
Fred