Most likely the Top Flite muffler was intended for a 60 size engine. The resulting low volume for a 90 size engine will cause the 91FX to have high combustion temperatures.

You will most like noticed this as your plane flies by and the engine seems to be missing like the ignition is being interrupted. You will not be able to tune this miss out of the engine.

Bisson makes a nice Pitts style muffler with adequate volume.

Enjoy,

Jim