When attempting an outboard motor repair, the main thing to remember is that fuel, spark, and compression are all required for correct operation. The fuel for an outboard motor is typically gasoline, and it is combusted to drive the pistons up and down. Spark is the ignition source that allows the fuel to combust, and compression refers to the integrity of the cylinders that is necessary for the pistons to move efficiently. Since each of these components can interact in potentially unexpected ways, outboard motor repair typically requires that each be tested and verified. Fuel problems can often be traced back to the ignition system or poor compression, especially if the cylinder responsible for sending a pulse signal to the fuel pump is not working properly.
There are many different types of outboard motors, so the repair process can differ somewhat depending on the design. If the engine isn't running or is running poorly, a good place to start the outboard motor repair process can be compression. Each spark plug must be removed prior to checking compression, after which the testing device can be screwed into one of the plug holes. The engine can then be cranked or turned over several times until the needle on the gauge has stopped rising. If any of the cylinders are substantially lower than the others, that indicates there is probably a mechanical problem with the motor.
To further diagnose potential mechanical problems, a leakdown test can be performed. This involves screwing the leakdown tester into the suspect cylinder, rotating the piston to top dead center (TDC), and then pressurizing it with air. If the tester shows leakage, it is typically possible to listen at locations such as the intake, exhaust, and nearby cylinders to determine which component has failed.
If the compression is good, the next step in outboard motor repair is often to use a spark tester to check the ignition system. Spark should typically be tested for at each cylinder, using either a dedicated testing unit or test light to reduce potential fire hazards. If any of the cylinders do not have spark, components such as the magneto, stator, and plug wires can be checked, depending on the particular make and model of the outboard motor. This can be one of the more complicated areas of outboard motor repair, especially for those without specialized knowledge.
When both compression and spark appear to be fine, the fuel system may be inspected. Fuel issues can be located in the motor, such as the carburetor, fuel pump, or tank. Common tank problems include a broken anti-siphon valve, bad squeeze bulb, and loose or poor fuel line connections. Each of these components can be inspected in turn to determine which is causing the problem.