Thanks for the replies so far!!! Keep them coming!

I did run two tanks of gas through the engine (upright) with the stock muffler with no problems for a "break-in period".

The Jtec muffler I have for it was purchased by the builder, not me. I have no way of knowing if the Jtec muffler is made specifically for the OS .91 FX. I'm going to try running the engine in the plane upright as fast'n'low suggests and see what happens. I will then also try putting the stock muffler back on it to see what happens with it.

I have always owed a OS engine, I have owed several OS .46, .61, and .91 FX engines. This is the first one to give me major trouble!!! Thanks for the suggestions!!!