Try the stock muffler! I had an O.S. 1.60 that would not run on a Jtec muffler. Maybe another brand would work. My engine would run on the ground with a long reach Fox plug just fine. With an O.S #8 it would overload and die at midrange sometimes. I have never had an engine give me so many dead stick landings. My new Ultra Sport 1000 still lives, but looks like it has hail damage from the trees it landed in :{ It was a muffler problem. Good luck!