Ever had to cancel a flight because of an excessive magneto drop? Most of us have. It’s probably the most common cause of going home instead of going flying, except perhaps for weather. I won’t insult your intelligence by going over the various causes of excessive mag drop. However, have you considered that maybe there is nothing wrong?
Check out this Lycoming Service Instruction.
Lycoming says that if you experience a higher than expected magneto drop at full mixture, to lean the mixture and try it again. They are not referring to the “burn off” procedure your instructor taught you. This is simply to improve the combustion efficiency and thereby reduce the effect of turning off half of the spark plugs.
“(d) If the RPM drop exceeds 175 RPM, slowly lean the mixture until the RPM peaks. Then retard the throttle to the RPM specified in step E.(1)(a) or E.(1)(b) for the magneto drop-off check and repeat the 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, and the engine is running smoothly, then the ignition system is operating properly. Return the mixture to full rich.”
So next time, when you experience a higher than expected mag drop, but the engine still runs smoothly on the suspected mag, try this. Maybe it will result in going flying instead of going home.