Re: 1948? Jhonson seahorse 16hp
I agree 100% with Daviet in that the SD-15 motor is a 1946-48, and that they were designed to run a 16:1 mix of straight 30 oil or 24:1 TCW oil. The SD motors were the first of the streamliner motors when they were introduced as the SD-10 for 1940. They were also the first to use a pressurized fuel system. The large built-in fuel tank on the cowl had a significant portion of its capacity below the float level of the carb so they pioneered the pressurized system on that motor. Later that system was used to pressurize a remote tank starting with the 1949 QD-10 (10 horse) & the restyled but essentially carried over 1949 SD-20 (16 horse). The SD-15 had a strengthened lower unit casting, a heat shield on the back because of the possibility of the lower unit getting hot from exhaust, and an additional water inlet at the front (those pressure-vacuum cooling systems needed all the help they could get). At least there is no impeller to replace. No neutral or reverse either. You needed to be a real man to operate one of those motors competently, basically start in gear and hold on!
The 1946 SD's heat shield was painted, but the 1947 through 1950 models left them unpainted (discoloration due to heat).
Although there is no impeller to worry about, you need to make sure the gearcase is always water-free. They were less than watertight if you ran them wide open or in choppy waters. That lower unit & gearcase ushered in the era of better things to come, starting with the QD-10, but the SD lower unit was never used again after 1950. Things had evolved. Also, the proper sealed gas cap needs to be in place or you will have no fuel pressure. Don't lose the cap!
