O.S., like a lot of engine makers, suggests propellers that will allow the engine to run well and also give low noise levels. In Japan, and many other parts of the world, modelers are under heavy noise restrictions. Larger, low-RPM props help keep noise down. That's why you see such large props in the instructions.

The O.S. Max .75 AX can easily turn in the 12,000's. Propping it for 12,000 to 12,500 RPM when leaned to just rich of peak RPM on the ground won't hurt it a bit, and will really unleash some power. You'll also get quite a bit more noise than when you use a recommended propeller. We'll use a 12 x8 or even an 11 x 9 or 10. Try an APC prop, too. Different makers have different airfoils and profiles for their props, which can make for widely-varied results, even when the props are supposed to be the same size.