As far as I know is 50:1.
50/1
The 1978 model, 115 hp Mercury outboard requires a 50:1 ratio, one pint of 2 cycle oil, to six gallons of fuel.
The 1979 - 1980 model, 115 hp Mercury outboard, requires a 50:1 ratio. One pint of 2 cycle oil, to six gallons of fuel.
The 1976 model, 115 hp Mercury outboard, requires a 50:1 fuel to 2 cycle oil ratio. This is roughly one pint of 2 cycle oil, to six gallons of fuel. The use of 4 stroke type motor oil is not recommended, nor intended, for fuel / oil mix.
Set the gap at .035 in. for a 115 Mercury.
If not using oil injection, the 115 hp Yamaha outboard will require a 50:1 ratio.
The dry weight of a 1988 model, 115 hp Mercury outboard is 312 lbs.
Mercury outboard 115 hp, serial number 2931955, is a 1970 year model.
The 115 hp Mercury outboard, serial number OD049801 is a 1991 year model.
A 115 hp Mercury outboard, serial number OC238909, would be a 1990 year model.
If running without oil injection, a 50:1 ratio is required, roughly one pint of 2 cycle oil, to six gallons of fuel.
I would assume 50:1 but call the manufacturer to confirm