What do you recommend for the best 3D prop on an .50SX using an FuntanaS .40.. I have an 12.25x3.75 but feel it may be too small for the engine??

Try a APC 13-4W on a 50 for better hovering. If you want a faster spin up, go with a APC 11.5-4. Between those 3 props you should find something your happy with.

That should be a good prop for the motor. What RPM is it turning?

I found the 13 x 4 slower to spool up with. The 12 x 4 apc is what works best on the 50sx.

I tried several different props with my .50SX/Pizzaz combo. I agree the 12x4 APC seemed to work best.