The YS engines don't use muffler pressure to their tanks, (they use crankcase pressure to pressurize the tank and a fuel regulator instead), so they don't "need" a muffler at all other than to keep the nosie down. Like the others above, I found that many of the early four stroke only had a straight pipe with no pressure tap (my Enya .60 four stroke is this way) and they ran fine on just carb suction. I think the fact that many of them also have pretty conservative carb throat sizes also helps create good suction/fuel draw, even without muffler pressure.

Lee