I am running the Bisson pitts and have no problem getting my mixture rich enough if need be. I do not recommend pinching off one of the muffler stacks as that will reduce your power output. I AM running the Du Bro LARGE clunk and LARGE fuel line to the carburetor. I am at 3000 ft. in altitude as well and that may be why I have no problems getting the mixture rich enough but not sure. Other guys at my field have no problem getting this engine/muffler combination to run rich.