Spark Plugs for Outboard Motors

Spark plugs for outboard motors

- How they work -

A spark plug, as the name suggests, is used to create an electrical spark which is used for igniting the fuel-air mixture in the combustion chamber. Without spark plugs, the engine won't start.
The ignition voltage generated by the spark coil and timer (ignition contact - today an electronic ignition system) is transmitted to the spark plugs via the spark plug cables. There, the ignition spark generated by the ignition voltage jumps between a ceramic-insulated center electrode and an earth electrode fixed to the mounting thread, thus starting the combustion process.

Self-cleaning

To prevent complications, spark plugs have what is known as self-cleaning. This only takes place at certain temperatures. The spark plug should quickly reach its self-cleaning temperature between 360 and 850°C. Self-cleaning prevents a short circuit or a shorter spark gap between the electrodes by burning off combustion residues.

Heat range

Spark plugs have a certain heat value which must be adapted to the respective engine. The heat range is affected by the length of the insulator nose surrounding the centre electrode. With a short insulator nose, heat can be quickly transferred to the cooled cylinder head via the thread. This is referred to as a "cold" plug with a high heat value. Spark plugs with short insulator base are used for engines that are subject to high demands. If the heat value is not suited to the motor, malfunctions may occur. If the spark plug is too "cold", short circuit can happen. If, on the other hand, the plug is too "warm", the electrodes will burn too quickly.

Reasons for changing spark plugs

Spark plugs must be replaced regularly due to electrode wear. Change interval is specified by the manufacturer and can be found in the manual. However, sometimes you may have to change spark plugs beforehand. You can see when, if the gap between the two electrodes is too large, as this prevents the spark from jumping over and the engine from starting. A change is also due if the spark plug is too dirty / sooty and cannot be cleaned sufficiently. Similarly, if your engine doesn't run smoothly or is unsteady. Or if there is a noticeable loss of outboard power and increased fuel consumption. You might also notice something not quite right about the exhaust gas values of your outboard motor.

If any of the above are apparent, you should check your outboard spark plugs and avoid damage to your engine. You can find out how to maintain & change your spark plugs in our guide to Winterising your outboard. 
Find solutions to common outboard problems and what you can do about them in our guide Fixing outboard problems.