Muffler or no, running lean is caused by the reluctance of some pilots to ever touch the carb needles..

We run our methanol GT80 racers at 11,500 rpm with only a 1 inch stack to keep from burning holes in the cowl..
Only hole I ever saw in a piston was a Q75 race engine running about 50% nitro in the fuel...Couldn't get it rich enough....The carb fuel passages weren't large enough....[8D]