Hello Airbusdrvr,
My 61SF runs beautifully on a muffler. It even idles at 1500 RPM!
On a muffler, it is a better torquer, because it is relatively low-timed for low RPM pipe use.

YS never made a "stock" muffler for this engine, but Hattori did.
It was a plain expansion chamber: not much good at making the engine quiet.
Look to the aftermarket like Jett or Performance Specialties.

best regards,
Dean Pappas