from MarineMechanic.com

 (for some reason, this excellent site is down at this time.  Here is the archived copy of this essay from Google)

OMC's "Engine Tuner" and Mercury's equivalent "Power Tune" can do a bang up job of cleaning carbon and oil build up from the piston, piston rings, and cylinder heads of 2 stroke engines. Depending on your choice of fuel, oil manufacturer, and idling habits, the recommended application of these cleaners, is at least annually. Those who troll a lot should shorten the time frame. But don't get carried away and always carefully follow the instructions on the can . It's very potent stuff and will remove paint and damage some plastic parts if left on too long. It also can be used as a carb parts cleaner for a carb you had to remove for cleaning.

The problem of carbon build-up from today's fuels is much worse than it was a decade ago, and not getting any better. Basically, these "tune up in a can" products contain highly concentrated detergents, similar to what's found in today's gasoline.

These tune up in a can products contain detergents that are carried in a solvent, and since the solvent is a petroleum distillate, both components actually help break down varnish, carbon, and other deposits in the combustion chamber. This cleaning action frees-up pistons and rings that have been gummed-up with the carbon deposits left behind by the gasoline.

When the rings become clogged, the carbon acts as an insulator that doesn't allow heat to transfer from the piston to the cylinder wall. If it gets bad enough the rings can crack, scuffing the inside of the cylinder walls, letting pressure bypass and reducing compression. Once this process starts, it gets worse quickly. Because the heat can't transfer from the piston to the cylinder walls, the piston expands and seizes in the cylinder.

These cleaning products are simple to use and can be used in both 2 and 4-cycle engines. Tune-up in a can products work much like oven cleaner. You heat the item to be cleaned then apply the cleaner , allowing both the heat and the chemicals to do the job.

Note: Never run a marine engine without water.

Warm the engine up to normal operating temperature. Try to maintain at least 1,200 RPMs during the spraying process. If the engine has more than one carb, distribute the fluid evenly between the carbs. You may have to fiddle with the throttle a little to keep the motor from stalling. Also, be sure to wipe up any drips or over-spray. If it can remove carbon, it can remove the paint job from your motor too.

Once you have sprayed the recommended amount into the engine, stall the engine with the spray and turn the key off. In colder climates put the cowling back on, to keep the engine warm. Soak for 20 to 30 minutes to allow the chemical cleaners to do their work. Then start the engine as usual and run five to ten minutes between 1000 to 2500 RPMs. It is generally best to lake run the engine under load as soon as possible to help remove all of the cleaner and goop the cleaner loosened up.

Large engines require a whole can of cleaner but very small engines probably only need 1/8 to 1/4 can to do the job. The way to tell is when you are getting a heavy white smoke from the exhaust, you probably can spray a little more, then let it stall. (My usual rule of thumb for current model 9,10,15,20,25,30 hp two cylinder outboard motors is a 1/2 can of cleaner, give or take.)

The real trick is to use the product regularly, this holds especially true if you use your motor for trolling. It really doesn't matter which manufacturers brand, they all seem to work pretty well.

By the way, OMC makes a special injection-style of its Engine Tuner, which attaches to a tire-valve type fixture on late model OMC outboards that have a separate electric fuel primer, instead of a choke. Ask your servicing dealer about it.

Ways to reduce carbon build up in your engine.