My OS heli carb does not fit a ST G90 and is not bigger on the inside. At the time I had the G90's the OS carbs were priced out of site.

The Magnum 108 carb fits the ST G90 like it was made for it. The replacement carb comes with a spring steel insert to reduce the venturi. Even with this reducer the venturi is still larger than a ST.

I ran my G90's with APC 14 X 6 props and the Magnum Carb with the venturi insert and they turned 11,000 rpm. with the genuine ST G90 carb they would only do 10,500 with the 14 X 6

I also ran 15 X 6 and 15 X 8 props on them. The engine did as well or better with the ST carb. with these big props. The engine did not over heat but spool ups were slow.

With a 13 X 6 the G90's seemed to be running too fast for the torque and power curves. The 13 X 8 was acceptable. All props were APC's

The S90K with the four bolt head or the S90 with the machined dished 6 bolt round head may do better than a G90 on a 13 X 6 because on these engines, the internal port passages are larger