Re: 6 hp evinrude early 80s
Why would you change the oil mixture? That has nothing to do with how the engine runs. Leave it at 50:1.
However, the high altitude definitely affects the mixture between air and fuel, which is what was meant. There's less oxygen in the air at altitude, and the air is less dense. That makes for a richer mixture. (too much fuel...not enough oxygen) Try adjusting the slow speed needle slightly clockwise. That will lean the fuel out just a little bit, and may fix your surging. Not too much, mind you...it's not that lean. In 1/8 turn increments, turn the needle with the engine idling. When it gets to the point where it's running at maximum idle, it's a little too lean, so back it off a little.
Then, when you're back in lower altitudes adjust it back for that.
The folks answering you here know what their talking about. Leave the fuel

il ratio alone, or you soon will have no engine to use at all.