Outboard motors are detachable engines that are mounted onto brackets located on the back end of a boat or on a boat's stern. Outboard motors are internal combustion engines that drive a single large propeller. The propeller rotates and pushes the boat through the water. A lever on the top of the motor changes the propeller's direction and is used to steer the boat. Like all internal combustion engines, outboard motors need maintenance and repair. The most common problems found in outboard motors can be detected with a multimeter.
Examine the fuel tank to make sure that it has enough fuel.
Remove the spark plug cover of the outboard motor. Take off the spark plug wires. Unscrew the spark plugs and set them aside. Examine the spark plugs. The bottom of the plug should be wet with gasoline. If they are not, there may be a blockage in the fuel system. Check the fuel line or fuel pump if you have a remote tank configuration. If the spark plug is covered with black carbon or oil, it may not be sparking properly. It could be that the spark plug that is in the motor is not the proper one and it is being used in an engine that gets too hot for it to work. It could also be that the engine has been running for long periods of time with the wrong amount of fuel or fuel and oil mixture. If your engine has more than one spark plug and only one of them is dirty, the cause is a weak spark from the ignition system.
Check the spark plug operation. Clean off the old spark plug or plugs with the pointed end of a safety pin, scraping off the carbon deposits that built up on the insulator that covers the centre electrode of the spark plug. Be sure to scrape between the insulation and the sidewall of the spark plug to remove as much carbon as possible. Pour a small amount of paint thinner into a plastic container. Put on rubber gloves to protect your hands and swirl the spark plug around in the container. Wipe off the spark plug with a paper towel. Set it out to air dry.
Look at the hooked wire on the end of the spark plug. There should be a gap of .025 inch between the wire and the end of the body of the spark plug, which is slightly thinner than a credit card, about .030 inch. Connect the plug wire to the spark plug. Set the plug onto the engine block in a position where you can see the gap between the hooked wire and the spark plug body easily. Try to start the engine and watch the spark plug. A spark should appear on the hooked wire of the spark plug. If there is no spark or it is very weak, this means that the coil inside the magneto of the engine or the breaker points of the engine need to be repaired.
Check for a continuous path for the spark. Turn your multimeter on. Set your multimeter to the ohms setting. Select the 40 ohms range. Leave the wire attached to the spark plug. Place the negative or black probe of the multimeter onto the engine block. Insert the red probe into the spark plug socket. The reading display should read somewhere between 3 ohms and 15 ohms. If it is higher, there is a poor connection in the spark plug socket. If the display reads OL, it means that there is an open circuit or a break in the electrical path. Take another reading with the multimeter to ensure that this measurement is correct. If you have a 2-cylinder, 2-cycle engine do the continuous path test for each cylinder.
Hold the flywheel in stationary position with a wrench and remove the large nut that holds it to the crankshaft. Loosen the flywheel and remove it. Find the breaker points of the magneto ignition that are located under the motor's flywheel. Breaker points are contact surfaces. Look to see if they are clean and shiny. If they are pitted they will need to be replaced. If they are dirty, scrape off the grime with a sharp carpet-layer's knife. Wipe the breaker points off with a paper towel that has some paint thinner on it.
Disconnect the condenser lead of the outboard motor. Change the setting of the multimeter to capacitance function. Touch the capacitor case with one probe and the terminal that it is plugged into with the other probe. If it reads .015 to .030, the condenser is fine. If the display reads OL the capacitor needs to be replaced.
Wear work gloves when handling sharp objects.