What does no RPM drop during a magneto check suggest about the left magneto?

When performing a magneto check, a no RPM drop during the test typically indicates a problem with the magneto system. Specifically, if there is no reduction in engine RPM while switching to the left magneto, it suggests that the left magneto is not delivering spark to the engine cylinders as it should.

In normal operation, when the magneto switch is changed from the "Both" position to either "Left" or "Right," you should see a slight drop in RPM due to the fact that the engine is momentarily running on only one magneto. If there is no RPM drop when switching to the left magneto, this suggests that the left magneto is unserviceable or that it is not providing the necessary ignition to the engine.

This scenario could arise from several issues such as a defective magneto, a malfunctioning lead, or other failures within that specific ignition system. Hence, the observation of no RPM drop is indicative of an unserviceable left magneto.