Ignition coil & CDI

How it works

The ignition coil turns the bike's low voltage into very high voltage to make the spark at the plug. The ignition unit (a CDI on many small bikes and 2-strokes, or the ECU on fuel-injected bikes) decides the exact moment of the spark, using a sensor that reads crankshaft position. On some bikes one coil feeds two plugs; on others each plug has its own stick coil pushed straight onto it. A tired coil gives a weak spark: misfiring under acceleration, hard starting when hot or in damp weather, a dead cylinder. Plug leads and caps can also crack and leak high voltage, especially in the rain. The CDI rarely fails, but a corroded connector can cut the ignition intermittently. Diagnosis always starts with the plug and cap, then measuring the coil's resistances with a multimeter against the manual.

Maintenance tips

• ✓If it misfires, start by checking the spark plug, the plug cap and the HT lead.
• ✓Measure the coil's primary and secondary resistance with a multimeter and compare with the manual.
• ✓Check the plug caps are fully seated and uncracked, especially if misfires appear in the rain.
• ✓Clean and protect the ignition unit's connectors.
• ✓Never run the engine with a plug disconnected and not earthed: it can damage the coil.