The motor is a ST90 with a OS 60F carb installed for testing.

The muffler is a GMS .60 size muffler ( same as a towerhobbies ). I made the adapter. With this muffler the ST carb did the typical blubber rich at low mid range, to correct this I had to re-machine the slot in the carb barrel to change the fuel delivery profile. The ST carb now works excellent, I can run a hot plug or a cold plug, it just runs fine. I didn't measure the performance with the stock muffler so I have no side by side data, but it shows no gain when compared to another ST 90 I have with the stock muffler. This was suprising because a GMS 1.20 muffler on a ST2300 gave me a significant RPM gain.

Beings this was a new motor with low run time I am using it to do some experimenting and testing. When I get more results I'll post them in a new thread ( assuming I have positive result, HaHa ).

I included some pictures for the adapter and carb barrel