I first used the OS 46AX with 10% nitro, #8 glow plug and a 10x6 APC propeller.
It pulled my calmato trainer incredibly well and even reached a top rpm of 14000, but i richened it down to 13500rpms and I was very happy with the result.

Following the advice of many here, I switched to an 11x6 propeller which is supposed to be the ideal prop for this engine.
Now All I can get with 5% or 10% is 10500 to 11000rpm max. It is still pulling my plane but I am not having unlimited verticals with the Sig 4star and it is quite slow for my taste (I moved the engine from the trainer to this one)

I tried a new #8 glow plug and still had the same result. Tried an 11x7 and 11x8, it was faster but loaded the engine even more to the point that I barely reached 10000rpms.
I don't know if something is wrong with my engine, but I wouldn't even think of putting a 12x7 prop on it.
Maybe I should increase to 20% nitro?

I've tried zinger props, APC props, MAS props, and finally top flite which gave me the fastest rpms for the 11x6 size.