Ignition Coil

The ignition coil is a transformer that turns the low voltage of the bike's electrical system into the very high voltage, typically tens of thousands of volts, needed to jump the spark plug gap and ignite the air–fuel mixture.

It has a primary winding with few turns and a secondary winding with many. When the ignition system suddenly interrupts the current in the primary (or, in a CDI system, discharges a capacitor through it), a high-voltage pulse is induced in the secondary and sent to the plug. Older bikes have one coil per cylinder or a twin-spark coil; modern ones often use stick coils fitted directly on top of each plug.

A failing coil causes misfiring, especially when hot or under load, hard starting or a dead cylinder. Check plug caps and HT leads for cracks too, and measure coil resistance against the manual's values.