ORIGINAL: Fire Bolt
I would use a 11x4 APC prop.
A agree with Firebolt but try an APC 11.5-4 also. I have that running on an OS 40FP and it turns it great. Probably even better on a 40FX.
The 12.25-3.75 is WAY TO MUCH!!! My 46FX can barely turn this prop. Very sluggish. This prop is better for 50 sized and above engines.
Joe