CDI ignition

CDI (capacitor discharge ignition) is the ignition system used on most scooters, motocross bikes, ATVs and many smaller motorcycles. Inside the CDI unit a capacitor is charged to a few hundred volts and, at the right moment, discharged into the primary winding of the ignition coil; the coil raises this to tens of thousands of volts and the spark plug fires.

Timing comes from a pickup sensor reading the position of the flywheel or alternator rotor. Many CDI units also contain the ignition advance curve and the rev limiter. AC-CDI is powered directly by the alternator, DC-CDI by the battery.

Signs of a failed CDI are no spark, misfiring when hot or a sudden hard rev limit. Before replacing the unit, check the plug, plug lead and cap, ignition coil, pickup and connectors – they fail far more often.