I'm running an OS .91FX, upsidedown, on 30% heli fuel, and a 15X4 W APC prop. I removed the baffle in the muffler. Before you tell me I'm crazy, I live in Denver, and the air pressure is lower here, whereas the inside muffler pressure is about the same. Fliers often reduce muffler pressure somehow to compensate for this. There is another fellow here at the same field with the same plane. Both of us experienced balky idle, with stalls at unpredictable moments. He rotated his cylinder to the side, installed a Pitts style muffler,and his problems went away. I'm thinking of doing the same, although I don't want to. Any thoughts or experiences?