Classic Bike Ignition Systems

Motorcycle Driver

sporty driver/Wikimedia Commons 

There are two common ignition types associated with classic bikes: contact points and fully electronic. For many years, the contact point ignition was the favored system to control the timing of the ignition spark. However, as electronics, in general, became more reliable and less costly to produce, manufacturers turned to fully electronic systems—cutting out the mechanical contact points.

The contact point ignition system consists of:

  • A battery or magneto to supply low voltage current for the spark
  • Mechanical contact points to control the point of ignition
  • A rotating cam to operate the contact points
  • A condenser to reduce arcing across the contact point surfaces
  • An ignition coil
  • A spark plug

The job of the ignition system is to supply a spark at the correct time within the cylinder. The spark must be sufficiently strong enough to jump a gap at the spark plug electrodes. To achieve this, the voltage must be increased considerably from the motorcycle’s electrical system (6 or 12 volts) to around 25,000 volts at the plug.

To achieve this increase in voltage, the system has two circuits: the primary and the secondary. In the primary circuit, the 6 or 12-volt power supply charges the ignition coil. During this phase, the contact points are closed. When the contact points open, the sudden drop in power supply causes the ignition coil to release stored energy in the form of the increased high voltage.

The high voltage current travels along a lead (HT lead) to a plug cap before entering the spark plug via the central electrode. A spark is created as the high voltage jumps from the central electrode to the ground electrode.

Contact Point Shortcomings

One of the shortcomings of the contact point ignition system is the tendency for the heel on the points to wear, which has the effect of retarding the ignition. Another shortcoming is the transfer of metallic particles from one contact point to the other as the current attempts to jump the increasing gap as the points open. These metal particles eventually form a “pip” on one of the point’s surfaces, making setting the correct gap, during service, difficult.

The construction of the contact points has one other shortcoming: point bounce (particularly on high performance or high revving engines). The design of contact points calls for spring steel to return the points to their closed position. As there is a time delay between the points being fully open and returning to their closed position, the high revs of performance engines do not allow the heel to follow the cam properly tending to bounce the contact faces apart.

This problem of points bounce creates a misplaced spark during the combustion process.

To eliminate all of the shortcomings of mechanical contact points, designers developed a system of ignition using no moving parts other than a trigger on the crankshaft. This system, made popular in the 70s by Motoplat, is a solid-state system.

Solid-state is a term referring to an electronic system where all amplifying and switching components in the system utilize semiconductor devices such as transistors, diodes, and thyristors.

The most popular design of electronic ignition is the capacitor-discharge type.

Capacitor-Discharge Ignition (CDI) Systems

There are two main types of current supply for CDI systems, battery, and magneto. Regardless of the power supply system, the basic working principles are the same.

Electrical power from the battery (for example) charges a high voltage capacitor. When the power supply is interrupted, the capacitor discharges and sends the current to the ignition coil which then increases the voltage to one sufficient to jump the spark plug gap.

Thyristor for Triggering

The switching of the power supply is achieved by the use of a thyristor. The thyristor is an electronic switch which requires a very small current to control its status or to trigger it. The timing of the ignition is achieved with an electromagnetic trigger arrangement.

The electromagnetic triggering consists of a rotor (typically attached to the crankshaft), and two-fixed pole electronic magnets. As the high point of the rotating rotor passes the fixed magnets, a small electrical current is sent to the thyristor which in turn complete the ignition spark.

When working with CDI type ignition systems, it is very important to be aware of the high voltage discharge from the spark plug. Testing for a spark on many classic bikes consists of laying the plug on top of the cylinder head (connected to the plug cap and HT lead) and turning the engine over with the ignition on. However, with CDI ignition, it is imperative the plug is ground properly and that the mechanic use gloves or special tools to hold the plug in contact with the head if a substantial electric shock is to be avoided.

Besides avoiding an electric shock, the mechanic must also follow all workshop safety precautions when working on electric circuits in general and CDI systems in particular.