Well, I thought that I would throw in my 5 cents worth here, I actually have the same engine in that position on a yak 54 of mine and also had a pitts muffler, and the one thing that I noticed is that the back pressure of the air rushing past the exhaust openings in flight becomes too great for the engine to compensate therefore it boggs and dies. This being my first larger engine I might be wrong, but when I changed back to the stock muffler the thing ran like a swiss watch.