Spark plug

The spark plug screws into the cylinder head and delivers a high-voltage spark across a small gap between its electrodes, igniting the compressed fuel-air mixture. Plugs differ in thread size, reach, heat range and electrode material, so always use the type specified for your engine.

A plug's appearance tells a lot: light brown or grey is normal, black and sooty points to a rich mixture or a lot of idling, white or blistered can mean a lean mixture or overheating, and wet oily deposits suggest oil getting into the chamber.

Change plugs at the service interval (iridium plugs last longer), set the gap to spec if adjustable, and tighten to the correct torque on a cold engine. Two-strokes foul plugs more easily, so carry a spare off-road.