I am using a 15% nitromethane mix. It is the same fuel I use with my OS .46 AX. The mfr recommends 0% to 15%. The manual does say that "[a]lmost all of the fuels on the hobby market with between 0% and 15% Nitro content will be fine, but you may get better results from a Premium fuel as opposed to a Sport fuel." (I don't know what that means.) It goes on to say that "[h]igher levels of nitromethane may require adjustment of the compression ratio to prevent detonation." I don't have the slightest idea how you adjust the compression ratio on an engine.

I did check very carefully this afternoon when I ran the engine. It seems to be better, but not 100% and there are no air bubbles in the fuel line.