What does a CDI unit do?

Answer 1

A Capacative Discharge Ignition unit holds the current from the generator, raises its frequency to thousands of volts and then fires it like a shot through the spark plug lead at precicely the right time to keep your motor running. A CDI unit incorperates a solid state logically programmable chip allowing unprecedented control over engine ignition timing throughout the engines entire rev range, as well as limiting the eventual speed of the motor under agreed conditions.

What could be thought of as a programmable electronic distributor.

Previously this job would have been done by a mechanical distributor/capacitor, with centrifugal weights and or a vacuum diaphragm advancing the spark on the power stroke.

A CDI unit promises to allow the motor engineers to time precicely when the spark plug will fire, at any point in the rev range, to provide the maximum power output under all predictable conditions. It also allows for efficient limiting of engine revs at the red line or in Top Gear.