LYCOMING SERVICE INSTRUCTION
Aircraft operators with Lycoming engines should be aware of Lycoming Service Instruction 1132B, issued June 18, 2010, which provides a procedure to use if magneto drop exceeds 175 RPM during the pre-flight check.
A basic magneto drop-off check based on engine speed is typically done during the before-takeoff check to determine the loss of engine speed that occurs when magnetos are switched from both magnetos to one (left or right) magneto. The maximum drop-off limit is in the Pilot’s Operating Handbook (POH).
Although this basic drop-off check, based on just engine speed, is satisfactory under most normal conditions, it does not take into account unusual circumstances of temperature, humidity and engine wear.
If the RPM drop exceeds 175 RPM, set the throttle to obtain 1,800 RPM. Slowly lean the mixture until the RPM peaks. Then retard the throttle back to 1,800 RPM and repeat the magneto drop-off check. If the drop-off does not exceed 175 RPM, the difference between the drop-off values for both magnetos does not exceed 50 RPM, the engine is running smoothly, then the ignition system is operating properly. Return the mixture to full rich.
From Cessna Direct Approach Magazine