I just spoke with Dave at Performance Specialties and he is very positive in his saying the UltraThrust(and JettStream) type muffler/pipes will be incompatible with the YS61. He said it would probably run wide open and at idle but nothing in between. He said back pressure would affect the diaphram pump. I'm still searching for a compatible muffler for the YS 61S Long Stroke. I really hate to have to go the Macs Pro way and pay $70 for their pipe($47.95 muffler, 10.95 adapter plus shipping). I really wish one of you guys who has a muffler lying around for the YS would send it to a good home. [8D][&:]