I run a Tower .75 with the same muffler as mentioned and have no problems whatsoever. Is your tank in the correct location. If so block off the air inlet on the opposite side as the head. The air will otherwise take the least path of resistance "not over your engine". I blocked oneside on my shoestring, and it dropped the incowl temp about 40 degree's. My clyinder head temp now runs about 225. Also make sure your outlet allows 2-3 times as much air out as in. My temp originaly was hovering in the 265 area, but I still have never had a dead stick or flame out. I am running the same #8 plug that I broke the engine in with and the plug was used then. I only run 10%, and I am on the fourth gallon this summer. I love the engine/muffler combo.

FYI,
The pitts from Bisson supplies plenty of pressure. This is an installation problem, not the engine or muffler. MY enigne runs @ 2-1/4 turns out, and I seldom ever have to touch it.