I am running a pitts style muffler on my DLE55 on the profile pictured in this thread. No problems for about 20 flights. I did use red rtv high temp sealant with the gasket that came with the engine.and let it sit overnight. Bought the pitts muffler from a seller here on RCU. Seller's username is ibcinyou. His name is Mike and is very helpful. He also sells the DLE's. I have heard of the side dump muffler causing problems. Maybe the way the exhaust is being direct or maybe a balance issue of the design of the muffler. My prop is balanced as well as the spinner. I would try the pitts style. Worked for me. Just some thoughts that seemed to work for me. Good Luck.