Please forgive my ignorance, I'm pretty new at the boat motor thing. I have 2 motors for my 12 foot tin, a 1954 Mercury Mark 20, and a 1982 Mercury 7.5. From what I've found, I should be using 50:1 for the 1982, but I'm uncertain about the Mark 20. My mechanic said 32:1. I found a doc on the web...