I am in the process of "revitalizing" a 1979 Yamaha 55hp 2stroke (duh) outboard engine. I have bought a service manual that covers alla yamaha motors built from 1984-onwards but i cannot seem to find and info regarding the gear ratio.
You don't really need to know the gear ratio; select the prop based on WOT rpm, and fine tune by other characteristics if necessary (hole shot, aeration in turns, etc.)
I will install a rpm meter the coming weekend and check the WOT rpm...i will post my findings for your valuable advice.
Till then, i have another question. The kill switch doesn't seem to function. The engine is able to start and operate without the kill switch attached. Where should i look to fix this issue?