When does a magneto ignition system generate a spark?

A magneto ignition system generates a spark when the primary circuit opens. This critical moment occurs due to the way magnetos work, specifically through the principles of electromagnetic induction.

In a magneto ignition system, a magnet is attached to a flywheel and spins as the engine operates. As the flywheel turns, the movement of the magnet induces a voltage in the coil. However, the actual generation of the spark is dependent on the interruption of the primary circuit, which typically happens when the ignition points open. This interruption creates a rapid change in magnetic field within the coil, leading to a high voltage in the secondary winding of the ignition coil.

At this point, the energy stored in the coil is released in the form of a spark at the spark plug, igniting the air-fuel mixture in the engine cylinder. While the other options describe scenarios related to engine operation, none directly relate to the mechanism by which the magneto creates a spark. For instance, the engine idling or the flywheel spinning rapidly doesn't inherently cause the spark; rather, it is the opening of the primary circuit that is the triggering event for spark generation in a magneto system.