I have a 75hp merc outboard and want to do a quick check of cables engaging etc but don’t want to burn up the motor.

I have Merc 500 50 HP outboard motor and have problems getting my 14 foot tri-hull to plane, it also sucks air sometimes, revving the engine.
Its an old boat and it was cheap so i don’t want to spend a fortune on fixes. I need some guidelines to find out if I might have the wrong lenght shaft. Also any pointers to do a cheap fix to lower the engine are greatly appreciated.