Joseph,

Try 12 -8 Master Airscrew, or 12-8 wood zinger and see how well they will
perform on your .91 These props will allow full rpm from your engine,
plus great speed with the #8 pitch. The wide blade MAS performs really well,
it is designed expressly for the four stroke.

I have four planes with .91 four stroke engines on them, they all run the 12-8
MAS, except the YS .91 uses a 14-8 Zinger.

FBD.