Last year I was working on the exhaust for my old scooter and had trouble getting the engine to rev up freely using two aftermarket mufflers for the smaller Briggs Stratton engines (3-5 hp).
At the time, I didn't worry too much about it since the duals setup was in development. I even put one muffler on my Snapper mower with no troubles. The other one got lost somewhere.
The other day I was working on the exhaust again trying to set up a pair of Harley style Fishtail exhaust tips and in my research, I opened up another small muffler to see what was usable inside. And I discovered the "advantage" of having trade agreements with China.
Inside the case was a short perforated tube held, in the middle, by a recess in a baffle plate . . .except there were TWO plates instead if just one. I checked the alignment of the plates by looking at the grinding marks from the disassembly and the holes were not lined up. The restriction in the muffler was probably 5 times higher than it should have been, if not more.
Now I suspect that is what happened on my exhaust previously, one aftermarket muffler was assembled wrong and the back pressure went through the roof because one small muffler couldn't flow enough under those conditions.
The warning is if anyone needs to replace a muffler on their lawnmower, watch out for those made in China. If the engine runs worse, try another muffler.
Btw, I'm running dual, re-cased (1 3/4") large style (5-12 hp) mufflers now and the pipes on the fishtail tips are long enough to give me a pair of bologna sliced tips for everyday use.
VBG