I have been researching this same engine tonight. I've gotten 1 reply and they say 25:1. I suppose this is because it is air cooled. Plugs are suppose to be NGK B7HS gapped at .024
I don't have the Mariner year model chart, but close as I can tell this engine would be a 1984 - 1985 year model.
internal tank model is 48#
The 1998 model Mariner outboards use a 50:1 ratio.
The 1998 model, 5 hp Mariner outboard requires a 50:1 ratio, three ounces of 2 cycle oil, to one gallon of fuel.
The dry weight of a 1986 model, 70 hp Yamaha outbord is 235 lbs.
i have a 25 hp mariner and i use a 50:1 mixture
A 75 hp Mariner, serial number OC282117 is a 1990 year model.
The dry weight of a 1985 model, 100 hp Mariner outboard is 312 lbs.
The dry weight for a 1993 model, 20 hp Mariner outboard is 112 lbs.
MARINER - ( 5 H.P. (1985)) Serial No: 00147151 Model No: 7005205 .
A 9.9 hp Mariner outboard, serial number OC269812, would be a 1990 year model.
A 150 hp Mariner outboard, serial number OB199335 would be a 1987 year model.