Re: prop size
Something isn't right here. Below is a post made by Don S that is pretty helpful. You should be turning much higher RPMs with that size of a prop. I'd check to make sure the tach is correct before proceeding.
Engine Won't Reach Operating RPM. Check
Fuel condition. Type and Octane possibly old fuel
Propeller pitch or diameter, damaged blades
Restricted fuel pickup tube or anti siphon valve
Fuel System Test
Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
Marine growth on hull and outdrive
Wrong gear ratio in outdrive
Restricted carburetor air intake (clogged flame arrestor)
Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
Poor cylinder compression
Compression Test
Carburetor defective, or wrong type.
Fuel pump pressure and vacuum
Boat overloaded, improperly loaded, or improperly trimmed.
Engine Overheating
Engine timing and ignition system operation
Remote control cables and linkage for proper travel to open throttle plates fully.
