The sportster engine is timed and compressed in such a way so it can turn big props. They are designed to be the 2-stroke equivalent to a 4-stroke essentially; "the poor mans 4-stroke". I measured the compression ratio at about 11.9:1 on my engines; most sport engines are 8-9:1. High prop load and high nitro on high compression engines is dead stick territory. My engine would get hot and quit using 15% and a 12x6 or 13x6. Drop to an 11x6 or 11x7 on 15% and it behaved better but it still got pretty warm. Dropped to 5% and it throttled much better. The combustion chamber is not shaped well for higher nitro either.
I settled on 5% nitro 25% castor fuel and it runs significantly better.