When I first bought my 50 used a few years ago it did same thing. Did some research and found that they will do that on occasion. The cure I found was change oil and filter, change spark plugs. Then just run it. Found out I had a bad plug and what those engines will do especially if you do a lot of low speed trolling with a bad plug will let that gas bypass the rings and go into crankcase. It also helps to run some seafoam through on occasion or Yamaha ring free. Since I figured that out I Change plugs every year. I pull cranks with big motor and haven’t had any more problems with it. And mine is a 04.
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Charlie Weaver USN/ENC 1965-1979