If, during a "Mag. drop" check there is no drop in RPM, what is the most likely cause?

In a "Mag. drop" check, the goal is to assess the performance of the ignition system by measuring the drop in RPM when one magneto is turned off. If there is no drop in RPM, it indicates that at least one magneto is not contributing to the ignition process. One of the switches being seized in the closed circuit position is a likely cause for this issue.

When a switch is stuck in the closed position, it prevents the circuit from opening when the magneto switch is turned off during the check. This means that the magneto continues to send spark to the engine, resulting in no change in engine speed (RPM) when the switch is toggled off.

The other options do not adequately explain why RPM would remain unchanged. A good ignition system would generally lead to a healthy drop in RPM when one system is turned off, while seized switches in the open position would stop current flow completely, likely resulting in a significant drop in RPM rather than maintaining it. Finally, if the plug leads from the magneto were not connected, the engine would not receive any spark from that magneto, leading to a noticeable drop in RPM during the check.