You have a valid point. I have a YS61(either Long or AR) with a tuned pipe on an Atlanta that just runs beautifully. The Cline system pressurizes the fuel tank via a one-way check valve in a line from the crank case. Then, the fuel to the carb flows thru the regulator much like a SCUBA tank system. I was assuming the YS crancase pump acted somewhat similarly and maybe I would have the same problem using the UltraThrust/JettStream muffler. In the particular application I am trying to remedy by replacing the OS91FX with the YS61SF, I will be unable to use a tuned pipe and must use the shorter engine mounted muffler.