It's almost never a good idea to lay an outboard motor down on its side or back. This is doubly true in the winter, when moisture from condensation needs to drain from the motor. Evinrude/Johnson emphatically states that four-stroke motors must be stored upright and recommends this for all other motors. Mercury Marine recommends upright storage to allow the water to drain from the motor, but Mercury also says that a motor may be stored on its side after the water has drained completely. Yamaha follows Mercury's recommendation, as does Mariner.
Pour a high-quality fuel stabilizer into the last tank of fuel before you plan to lay the motor down. Although it may seem like overkill, it doesn't hurt to use 1 oz. of fuel stabilizer for every gallon of gas every time you fill the boat's fuel tank. It prepares the motor and protects the fuel system from moisture damage if you must transport or store the motor on its side unexpectedly.
Disconnect the drain hose from the bottom-right side of the vapor separator tank and drain the tank into an "old fuel" can. Open the drain screw on the left side of the tank with a screwdriver and drain any water or fuel remaining. Close the valve and reconnect that hose. Disconnect the hose on the bottom-left side of the tank and open the drain screw on the right side of the tank, draining any remaining fuel and water. Tighten the screw and reconnect the drain hose. Remove the water separating fuel filter.
Connect a flushing attachment and garden hose to the motor, turn on the water and start the motor to flush the cooling system. Put clean oil into the oil tank to coat the bearing surfaces of the motor with a thin sheen of oil, helping to inhibit rust that may otherwise form in these areas. Allow the motor to run for between 15 and 20 minutes.
Shut the motor down. Disconnect the quick-connect on the fuel line between the tank and the carburetor and restart the motor. Spray fogging oil into the carburetor and let the motor run itself down. Twist and pull the plug wires free from the spark plugs . Remove the spark plugs from their bores with a plug wrench and spray the fogging oil into the cylinders. Replace the spark plugs; you're nearly finished.
Disconnect quick-connects for the fuel and oil tanks -- leave the oil hose connected to the motor, rather than the tank -- and lift the motor from the back of the boat. Lay the motor down on its side, carefully.
- "Evinrude Repair Manual -- 2.5 to 250 HP Models, 2002-2007"; Seloc Marine; 2007
- "Mercury Outboards Repair Manual -- 2.5 to 250 HP Models, 2001-2009"; Seloc Marine; 2007
- "Yamaha/Mercury/Mariner Repair Manual 4-Stroke, 1995-2004"; Seloc Marine; 2007