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This process will flush salt, dirt, and other contaminants out of your engine in order to prevent blockages and corrosion. Depending on what kind of motor you have, there are a variety of ways to perform a flush.  For older outboard motors, get a pair of boat engine “ear muffs” and attach them to the water intakes on your engine. Attach a water hose to the opening on the ear muffs, turn on the water, and let the engine run in neutral until the water runs out clean. Some newer outboard motors have built-in water hose attachments and a flushing system that can be used without running the engine. If you have this type of motor, you can attach a hose directly to the motor and let the water run for about 10 minutes. Check your owner’s manual, if you have one, for the correct flushing procedure. Unstabilized fuel can degrade during the winter, causing gummy buildups that can clog up your engine supply lines. Fill your gas tank with fuel to about 95% of your tank’s capacity.  Add a gasoline stabilizer, like Pennzoil Fuel Stabilizer, PRI-G, or Stabil. Follow the directions on the stabilizer packaging to determine the appropriate amount to add. Run your engine for 10-20 minutes after adding the stabilizer in order to distribute the stabilized fuel. Alternatively, you can drain your gas tank and supply lines completely and leave your tank empty over the winter. Coating the inside of your engine with fogging oil will help prevent corrosion. Check your owner’s manual or engine manufacturer’s instructions to determine the best type of fogging oil to use and the proper procedure for fogging your engine.  For some types of engines, you can simply spray fogging oil into the engine air intake while it is running. Spray a generous amount of fogging oil into the intake, then disconnect the fuel line. Continue spraying fogging oil into the air intake and allow the engine to continue running until it dies. The engine will probably put out a lot of white smoke during this process. Alternatively, if you have completely drained the fuel from your engine, you can remove the spark plugs and spray fogging oil directly into the spark plug holes. Spin the engine by hand several times to coat the spark plugs. Put the plugs back in but do not connect the wires. This will keep your boat's pistons from being subjected to air, dampness and other caustic materials when not in use. Using antifreeze will prevent damage that can result from water freezing in your engine block. Antifreeze containing propylene glycol is environmentally friendly and recommended by nearly all manufacturers. Use the highest concentration of antifreeze available (-100). The procedure will differ depending on whether you have an inboard or outboard motor.  If you have an outboard motor, you can connect an antifreeze kit to your engine’s water intake after flushing with fresh water. Leave your engine running after performing the fresh-water flush, disconnect your water hose, and connect a hose attached to a tank of antifreeze to your water intake instead. Let the engine run while connected to the antifreeze tank until the tank is empty.  For an inboard motor, take a large bucket of antifreeze (usually about five gallons) and insert the end of the water intake hose from the seacock into the bucket. Let the motor idle until you see antifreeze coming out of the exhaust outlet for at least 30 seconds. Replace the intake hose in the seacock. If you have an inboard motor, you should change the oil in your engine and transmission. This works best right after running the boat, while the oil is still hot. Hot oil flows more easily, and any contaminants or impurities will be suspended, making it easier to remove them. This is also a good time to replace your oil filter.  Check your owner’s manual for recommendations on which type of oil to use. Engine oil is available in a variety of viscosities and service ratings, and different engines require different oil properties. If you can, drain the old oil by removing the sump plug and allowing the oil to run out into a drainage pan or a cardboard box lined with a garbage bag. If this method is not convenient, you can pump the old oil out with a hand pumped or electric oil extractor. Some oil changing devices can both extract the old oil and refill your crankcase with fresh oil.  Dispose of your used oil at a recycling facility. This will flush out water and other contaminants that can cause rusting and corrosion. Get your engine hot before draining your gear case lubricant, as this will help the lubricant flow more easily and stir up any settled contaminants. If the lubricant looks cloudy or if you see metal shavings suspended in it, it may be time to get your gear case serviced. The battery should be fully charged when put away. Retain the charge and maintain the water level while the battery is in storage.

Summary:
Flush the engine with fresh water. Stabilize your fuel. Fog the engine cylinders and carburetor intakes. Flush your engine block with antifreeze. Change the oil. Drain and replace the gear case lubricant. Disengage the battery and store it in a safe, dry place for the winter.