Begin winter layup by stabilizing the gasoline, thereby halting its degradation into gum and varnish. Otherwise, by springtime the carburetor may become so plugged up the engine won't start. This procedure is even more important nowadays because gasoline shelf life has become so short: After ninety days untreated fuel turns sour. The exact quantity of stabilizer (ounces )required depends upon two variables: How many gallons of gasoline need to be treated and for many months. The larger the volume of fuel and the longer the spell of time, the more ounces of stabilizer required. Follow the instructions printed on the can, and wear rubber gloves.
Start the engine. Running the motor at fast idle circulates treated fuel throughout the filters and the carburetor, or in the case of an EFI motor, its injectors. Once the engine is warmed up, shut it off. If you're working on a stern drive, inboard, or four stroke outboard motor, immediately drain the crankcase oil so it's hot. The reason why is as simple as the sea is salt.
With all the acids and corrosive elements held in suspension, they spill out along with the hot, dirty crankcase oil instead of etching vital ring and bearing surfaces during the winter months. At the same time you drain the oil, also replace the filter so that no contaminated oil fouls the clean, new lubricant. But before installing the new filter, first fill it. That way connecting rod and crankshaft bearings don't have to wait for the oil pump to first fill a dry filter before sending lubricant onward through the oil galleys.
Another tip: Dab a fingerfull of oil around the filter gasket to help it seal against the block. With the new filter in place and the oil-level topped of, restart the engine, check the filter base for leaks.
Backwash the cooling system with a garden hose adapter, flushing out salt, sediment, and rust flakes that would otherwise corrode the cooling system. When appropriate, remove seawater impellers. If you're unsure whether your model of inboard or stern drive would benefit from this step, consult the shop service manual. Also check to see whether the engine should be running or turned-off while backwashing.
Rust-proof cylinder walls by fogging their surfaces with a super-sticky oil. With the engine idling, shut off its fuel supply so its running on just the treated fuel in the carburetor float bowl. Just as the engine starts to die, rpm will climb. At this precise moment, begin furiously squirting fogging oil down the carburetor throat. Flood the engine with a mist of fogging oil until it stops running.
Next remove all of the spark plugs and dump about an ounce of fogging oil into each cylinder. I use an old-fashioned oil can to pump in the stuff. With the spark plugs back in place (finger tight) and the ignition disabled, crank the engine for a few seconds to distribute oil onto the piston rings and cylinder walls.
Loosen water pump, alternator and power steering drive belt, to lessen the tension on those component's bearings. Inspect the drive belts for cracks or frayed edges. Replace as necessary. On inboards and stern drive motors, remove the carburetor flame arrestor and soak it in kerosene. Air dry then re-install. An alternative that I prefer, spray it with carburetor cleaner from the inside out. In other words, blow grit out, rather than deeper into the metal elements.
Grease the steering linkage, shift cable, tilt/trim mechanisms, and swivel points. Greasing expels water preventing it freezing, expanding and breaking parts. Zerk fittings that won't take lube should be unscrewed and replaced.
Drain the gearcase oil. Otherwise, trapped water could freeze, expand and break the housing. First, loosen the bottom drain plug just enough so whatever liquid is in the gearcase trickles out. A few drops of water followed by lubricant is OK. But more than an ounce and the gearcase should be pressured-checked to find which seal is leaking, so it can be replaced. The one ounce of water is not an arbitrary number. Gear oil blenders researched exactly how little water it takes to destroy a gear set.
Like an obsessive compulsive personality, examine the dirty oil for water, bits of broken gear teeth and bronze grit. Obviously these metal particles mean further repairs are necessary. Dip your finger in the oil and sniff it. If the odor is burned, further investigation is warranted.
Assuming oil is well, refill the lower unit with the factory recommended lubricant. Fill from the bottom so no air bubbles prevent completely filling of the oil reservoir. When lubricant spills out the top hole, the housing is full. Insert the top drain plug first. Make certain each drain plug has one and only one gasket, and that no gasket is lodged in the casting's recess.
Filling is neater when using a little handpump. One model attaches to gear oil tubes, another quart cans of gear oil. They make life simple.