Re: Maintaining a 1976 9.9 hp Johnson Seahorse
The water pump is the most important and the lower unit oil needs to be changed regularly as well. If it is left outside in the winter it should be done at the end of every season to ensure that any water that might have got in there is not allowed to freeze. If you keep it indoors for the winter (or above 32F) and only use it a few times each season it can be done every few years.
I would use NGK spark plugs. B7HS for regular use and B6HS if you intend to troll a lot. They will last longer then the recommended champion plugs, which are more susceptible to fouling and other intermittent issues, on that particular series of motor.
Every few years I like to pop the flywheel and reset the point gap to ensure the spark timing is correct. A 0.020" gap is the perfect setting. Overtime the rub bar wears and that gap will decrease moving your spark timing a little late and reducing top end performance. Every 3 years is probably OK for that unless you are having problems.
Always clean out the fuel filter screen in the fuel pump at the end of each season and make sure you run fuel stabilizer in it when the season ends, for storage. A little fogging oil if the storage is going to be longer then a year and of course a little of it doesn't hurt even if the storage is not that long. Spraying a little seafoam or engine tuner through the carb every few years will keep the cylinders from carboning up on you. 2 strokes are not the cleanest burning motors and a little decarbonation works wonders for the piston rings.
Other then that, you got a nice motor, it should work for another 37 years if you take care of it.
