A spark plug seals the combustion chamber. It conducts a spark that is generated in the ignition coil into the combustion chamber and provides a gap for that spark to jump across. Finally, it conducts the heat it picks up in the combustion process to the cylinder head and into the cooling system. Since 1936, NGK has built itself to be the world's leading spark plug manufacturer. NGK's spark plugs have a triple-gasket sealing process which helps them to withstand the roughest ride. A solid copper core increases spark and heat transfer and the longer insulating nose prevents fouling. The double dipped zinc-chromate shells resists corrosion and rusting.
Over time jet ski spark plugs can deteriorate from producing the spark. When this happens, it changes the spark plug gap causing irregular spark and missing. You don't have to wait to change your plugs. When you do change them, however, wait for the engine to be cold. Do notchange them when the engine is hot or warm because it could cause you to cross thread it by putting a cold spark plug in an heated up expanded hole.
