If it still has the original head gasket you might consider removing it,------The new gaskets are improved.---Sierra is the best deal.----Inspect thermostat as they are often stuck open or missing.
The original mix was 24:1 on those.------But 1961 was the first year for needle bearings on big end of the rods.----So internally there is no difference with a 1965 model ( now rated at 6 HP ) that ran on 50:1 mix.----Another lively oil debate.
I do not see a 75 HP listed for 81.----Serial # of your motor ?----Copper color decals ?-----I think an o-ring fits in that groove.----Best to get this correct as mistakes will be expensive.
It likely wants to jump into neutral at speed.----But the interlock prevents that.----Time to investigate.----On many motors the clutch dog only goes one way.----Big $$$ for parts.-----Lots available used.---Your location ?
