I have a 1968 Mercury Model 39 3.9 hp Outboard. What is the correct fuel/oil mix ratio?

Question

I have a 1968 Mercury Model 39 3.9 hp Outboard. What is the correct fuel/oil mix ratio?

Answer 1

Most of the older motors used a higher gas to oil mixture like 30 to 1 or 40 to 1.  That was due to the quality of oil available at that time.  You can safely use a 50 to 1 mixture as long as the oil is TCW 3  rated for marine use.  Do not use chain saw or any other 2 cycle oil.  The TCW 3 oils have superior lubricating properties that do not break down with the detergents in today's gas.  I have a 1947 Johnson 22 hp that  I have run for years with this mixture without any problems.

Answer 2

its not fuel to oil ratio its fuel to air ratio. A mixture is the working point that modern engine management systems employing fuel injection attempt to achieve in light load cruise situations. For gasoline fuel, the stoichiometric air/fuel mixture is approximately 14.7 times the mass of air to fuel. Any mixture less than 14.7 to 1 is considered to be a rich mixture, any more than 14.7 to 1 is a lean mixture

Answer 3

It is still 50 to 1.