At what moment does the spark plug fire in relation to the breaker points?

The spark plug fires at the moment the breaker points open. This is essential for the functioning of an ignition system in small engines. When the breaker points open, it interrupts the flow of current in the primary winding of the ignition coil, causing a rapid change in magnetic field. This change induces a high voltage in the secondary winding of the ignition coil, leading to the creation of a spark at the spark plug. This spark is what ignites the air-fuel mixture in the combustion chamber, allowing the engine to run.

Understanding this timing is critical for diagnosing ignition issues in small engines. If the spark doesn't occur at the right moment, it can lead to misfiring or failure to start. This synchronization between the opening of the breaker points and the firing of the spark plug is a fundamental aspect of the ignition system’s operation.