have an 88 Grumman pontoon 24' with a 70's Johnson 50hp. the motor runs at a lower rpm than I think it should, (no rpm gauge to know exactly) but Ive been around boats a lot and I know its not reaching peak rpm. Whats a good pitch to try and how can i tell if its in the proper rpm range? it has the large gear case I believe might be a 74' ill have to double check. boat does around 15 in smooth water/ light load. i bought the motor off of a little 15ft fiberglass fishing boat so i know its probably the wrong prop.