Whether two-stroke or four-stroke designs, boat outboard motors share basic engine components with car engines. Pistons, rods, crankshafts and valve stems all have to move freely within the engine to complete their cycles. Outboard motors can seize for multiple reasons, but mainly the lack of proper lubrication and oxidation prevail as the most common culprits. Overheating can seize engine parts. Long intervals of storage can seize engines. Whatever the cause, a few remedies and procedures work best for freeing up a seized outboard engine.
Disconnect the fuel line from the carburettor intake on the motor. Many outboard fuel lines line have push-pull quick release connectors. Use a screwdriver if your fuel line has a hose clamp connection. Disconnect any wire harness to the engine, such as a battery power supply or auxiliary gauges. Use a socket or screwdriver to disconnect the throttle linkage coupler. Use a socket to unscrew the nuts that hold the power tilt cylinders to the engine, if so equipped.
Unscrew the outboard motor mount twist clamps and pull the engine off. Use a socket to remove the bolts if your engine has a jack plate mount or other extension. Lay the outboard motor on to a hand dolly in a horizontal position with the spark plug holes at the top. Use bungee cords to strap the motor securely in place, and move it inside a garage. Unclasp the top engine cowl. Remove the spark plug wires and use a socket to remove all the spark plugs.
Attach a length of plastic hose to the tip of a clean turkey baster. Open a bottle of anti-seize lubricant and draw 15 ml (1/2 fl oz) of lubricant into the turkey baster. Squirt the contents inside the spark plug hole. Do the same for additional cylinders. You must keep the engine upright and let the anti-seize lubricant soak for a week. During the interim, you should add 15 ml (1/2 fl oz) of anti-seize lubricant to each cylinder twice a day.
Set the outboard motor and dolly in an upright position. Strap the dolly to a workbench post or garage support post with bungee cords, wrapped tight. Use a socket to remove the pull start mechanism bolts on the engine, or the ratchet assembly, and remove it so you can gain access to the flywheel centre nut. Equip an air or electric impact gun with a socket that will fit the flywheel centre nut. Turn the impact gun pressure dial to a medium setting, and for clockwise rotation.
Set the outboard motor gear in the "Neutral" position. Stuff rags inside the spark plug holes. Pull the trigger and let the impact gun shock the nut with the hammering blows. Use one to two-minute durations on the gun. Repeat the shocks for a dozen times or so. Note any movement of the flywheel. If the flywheel begins to turn, stop and add more anti-seize lubricant to the spark plug holes with the turkey baster and hose.
Let the lubricant soak for a few hours and try again. If the flywheel turns freely with the impact gun, you have unseized the engine. If the engine refuses to rotate, use a socket to remove the valve cover bolts and remove the valve covers. Position the engine in the dolly where the valve stems point in an upward position. Drip anti-seize lubricant on the top of the valve stems and let it seep into the valve guides.
Wet the valve stems three or four times within an hour. Take a rubber mallet and gently hit the top of the valve stems, particularly the valves in the "Open" position. Do this several times. If any valve stem moves, it means you have freed up a frozen valve stem. Use the impact gun to turn the flywheel nut. If the flywheel now moves, stop. Repeat the lubrication process on the valve stems again, and use the rubber mallet to tap them all. Try the impact gun again. If the flywheel moves freely now, you have unseized the engine.
Reinstall the starting mechanism, wires, spark plugs and engine top cowl. Remove the outboard motor from the dolly and then remount it on the transom. Reconnect any gauge wires, fuel line, tilt trim cylinders and the battery power supply in the reverse order that you removed them.
You must immediately change the oil and oil filter after this procedure, since the anti-seize lubricant will dilute and ruin the viscosity of the engine oil.
Wear gloves and a particle mask if you are sensitive to caustic fumes that might result from the use of the anti-seize lubricant.