I just ordered an OS 61FSR muffler from Mecoa. It is supposed to fit the YS. My fingers are crossed. Dave at Perf. Spec. said my OS91FX wouldn't run right with his UltraThrust and the Cline Regulator and he was right. I hate to gamble on another UltraThrust/JettStream for the YS61 when the guy that sells the muffler says it won't run right. However, if someone would send me one, I'll install and test it. I'm just trying to get this plane to run.