Outboard motor manufacturers recommend professional end-of-season servicing before the motor is stored for winter. Four-stroke outboard motors can be damaged if they are not serviced properly before storage, and failure to do so may also void any warranties.
If a professional prestorage service is not available, and the engine isn't going to be used for two months or longer, the most important thing to do is protect the motor from the effects of water that cause corrosion and rust. There may be water trapped inside the engine block and manifold system, and modern ethanol-blended fuels break down quickly so do not offer much protection.
Each outboard motor’s owner’s manual contains specific off-season storage instructions. Some winterization steps to perform include, but may not be limited to, the following:
Add fuel conditioner and stabilizer, then run the motor for a few minutes.
Flush with clean, fresh water.
Clean the powerhead and external surfaces.
Change the engine oil and filter.
Change the crankcase oil.
Change the lower unit oil and gear case lubricant.
Replace the plug washers on the lower unit.
Spray the engine with fogging oil. Use the built-in fogging ports if available.
Drain residual water from the water pump.
Remove spark plugs, spray fogging oil into the cylinders, then reinstall the spark plugs.
Drain fuel from the vapor separator tank, carburetor and primary fuel filter cup.
Install a new fuel filter.
Install a new water separator filter.
Fill the fuel tank completely to prevent condensation.
Grease internal and external fittings
Charge and disconnect the battery. Store it in a dry place.
Apply silicone engine protectant to the powerhead and any rubber parts.
Cover the motor and store it vertically.
It is important to note that some commercially available rust inhibitors are not suitable for all engines and may cause damage. The manufacturer-supplied owner’s manual should contain details of approved maintenance products.