1954 Mercury Mark 20 Oil Ratio

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 that seemed to suggest Mercury motors of that size and time period were 20:1. Anyone know for sure? What would happen if I ran the wrong mixture through the motor? I'd love to simplify and have one tank that works for both, any ratio that would satisfy both?

20:1 and it does not matter who says the oils of today are so good you can run it leaner. Stay rich and save the powerhead and tune out the air heads.

...finally found someone on my side...all I read is 50/1 or something bad will happen. Nothing yet..

About that Mark 20..I had a Hurricane 10 on a 12' v-aluminum boat and it did 24 mph with me only. Was on a wooden hydro before I bought it. The flywheel nut came loose while it was running. Did not know it until I stopped. Ran fine while loose. It destroyed the taper on the wheel and crank. If I were to get another similar motor I would maybe lap that taper and torque to specs. Or what ever proper procedure for it if lapping isn't. fyi..

Sinkingfast said:

About that Mark 20..I had a Hurricane 10 on a 12' v-aluminum boat and it did 24 mph with me only. Was on a wooden hydro before I bought it. The flywheel nut came loose while it was running. Did not know it until I stopped. Ran fine while loose. It destroyed the taper on the wheel and crank. If I were to get another similar motor I would maybe lap that taper and torque to specs. Or what ever proper procedure for it if lapping isn't. fyi..

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Thanks, will have to look into that...

Sinkingfast said:

...finally found someone on my side...all I read is 50/1 or something bad will happen. Nothing yet..

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Probably the most accurate knowledge base for vintage/antique engines and how to keep them alive and well can be found at. www.aomci.org.