The contact breaker (points) ignition system

 An essential part in any “elderly” motorcycle is the ignition system, operated via a “contact breaker” or “breaker points” ignition system.  Later motorcycles dispensed with the contact breaker in lieu of electronics, which we will cover in a later article.

The purpose of the ignition system is to ignite the combustion mixture in each cylinder at the precise time.  To achieve this, the following parts listed, and shown in the below diagram are connected together:

Contact breaker ignition system (Harley Davidson)
Image from my personal service manual collection, diagram of 1959-1978 Harley Davidson Sportster ignition system.
  1. The battery furnishes the electrical energy.
  2. The ignition coil and condenser transform the battery current to high tension current which will jump the gap in the spark plug.
  3. The breaker points opens and closes the primary circuit at the proper time.
  4. The spark plug provides the means to fire the air-fuel mixture in the cylinder.
  5. The wires connect the components together.
  6. The ignition switch is used to stop the engine.

An important concept needs to be stated.  When current moves or flows in a wire, it sets up a magnetic field around the wire.  You can concentrate this magnetic field and make it stronger by winding the wire into a coil.  And conversely, when a magnetic field moves, it can “induce” or move a current in a wire.

The “primary” circuit is connected from the battery positive terminal, the primary winding in the ignition coil, and to the contact breaker.  The cam shown in the diagram is connected to the rotating assembly of the engine, so it is permanently “in time” with engine rotation, and therefore the exact position of the pistons and valves inside the engine.

The ignition coil consists of two coils of wire – primary and secondary – within the plastic housing. The secondary winding has a great many more coils than the primary winding.  The exact number will be determined by the manufacturer for the specific motorcycle model.

While the contact breaker is “closed” or connected to earth, current is flowing from the battery positive terminal to earth.  The current flowing through the primary ignition coil sets up a magnetic field.

When the engine rotation reaches the proper place, the cam begins to rotate around and opens the breaker points, and the current flow from the battery to earth attempts to flow as the points gap opens.  Here is an exploded diagram of a Harley Davidson breaker points assembly.

Exploded view, contact breaker assembly, typical Harley Davidson
Exploded view, contact breaker assembly, typical Harley Davidson

The condenser has a very important function here.  The current flowing in the primary circuit would tend to arc across the breaker gap as it opens, and dissipate all the energy in the primary coil.  The condenser absorbs the current and brings the flow to a quick stop.

When the current stops quickly, the magnetic field in set up by the primary coil collapses. The moving magnetic field induces a current in the secondary windings in the ignition coil.  Since the secondary winding has a great many more windings than the primary, the induced voltage is very high, perhaps around 15,000 volts.  The secondary coil is connected to the spark plug.  This very high voltage is sufficient to make an arc jump across the spark plug gap.

The energy in the secondary winding dissipates as the spark plug fires, and the current stored in the condenser discharges back through the primary circuit. The process is ready to start again on the next ignition rotation of the engine.

Matt McLeod

About the Author

I teach people how to build custom motorcycles by helping them build skills and confidence with my coaching, articles and training videos. I provide better technical information for custom motorcycle builders. And I shorten the learning curve getting you there.


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