What does a high RPM drop during a magneto check indicate?

A high RPM drop during a magneto check signifies that there is a problem with the ignition system, specifically an imbalance in spark production. When a magneto is selected, it should maintain the engine’s RPM within a specified range. A significant drop usually indicates that one or more spark plugs are not firing optimally, which can be caused by fouling. Fouled spark plugs often lead to incomplete combustion or erratic firing, resulting in a noticeable decrease in engine RPM when the magneto is engaged. This decline reflects the reduced power output from the affected spark plugs, confirming that they are not providing the necessary spark for efficient engine operation during the check. Understanding this connection between spark plug condition and engine performance is crucial for diagnosing ignition system issues in aviation engines.