When does the spark appear at the plug electrodes?

The spark at the plug electrodes occurs when the contact breaker opens. In a typical ignition system, particularly in magneto-equipped engines, the contact breaker is a critical component that helps generate the high voltage necessary for the ignition spark.

When the contact breaker closes, it completes the circuit, allowing current to flow and energizing the ignition coil. As the engine continues to rotate, the contact breaker eventually opens, interrupting the current flow. This sudden interruption causes a rapid collapse of the magnetic field in the ignition coil, which generates a high-voltage pulse. This high voltage is then sent to the spark plug, where it creates a spark at the electrodes, igniting the air-fuel mixture in the combustion chamber.

Therefore, the moment the contact breaker opens is pivotal as it triggers the mechanism that produces the spark. Other options, such as the contact breaker closing or remaining open, do not lead to the generation of a spark at the plug electrodes. The role of the magneto switch pertains to powering the ignition system but does not directly influence the timing of the spark generated at the plug electrodes.