Does someone have an idea what the mix would be for an older Mercury 20 hp outboard. I think it is about 1980? My son is about to use it and has forgotten the ratio he used.
Just about everything uses 50:1. Some chain saws are 25:1 though. I once had an Evinrude 9.9 that was labeled 100:1 but the dealer advised they had a lot of them seize up and I should use 50:1. Ran fine on it, a little smokey, but they all are.
And use a high quality oil. it will smoke/pollute less as well as better protecting the engine.
I owned an outdoor power equipment business for 31 years. when customers would ask me if I liked brand x oil from the box store. I always replied "I love it. it's a guarantee you'll remain a customer in my repair shop".
These days, 50:1 is typical. But in those days 40:1 was the norm. Maybe oil is better and 50:1 with modern oil would be fine, but if it were my motor I'd use 40:1, if I couldn't find an authoritative source that said otherwise.
All my two stroke motors in the 70s specified 40:1. But, of course, that's not really the point. The point is that, in the absence of an authoritative reference for the specific engine in question, 40:1 is the safer choice.
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