Ah, the old ST .15…..I reworked many of these motors for FAI and, if I remember right, had them turning a 7 x 2.5 or 7 x 2.75 prop on FAI fuel in the 23,500 rpm range. I made a modified cox TD .15 head that was worth 1000-1500 rpm as well as other internal mods. I loved these things. In stock form, with some nitro, they can make alotta noise and run well. I would keep the prop in the 7 x 3.5 to a 7X 2.5 range (fiberglass or equiv.)
I had good luck with the grey or red insert g.15s but my best runner was the round venturied G.15. Again, all of my stuff ran on FAI fuel (because that was what I was into) but I am sure that 35-45% would make it wake up/ There were so many versions of these things that it was hard to keep track at times. I still like them, followed by the Rossi .15….
Run it and have fun!!!