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Magneto check

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Aircraft Systems, Commercial Pilot, Flight Instructor, General Aviation, Instrument Rating, Light Sport Aircraft, Private Pilot, Student Pilot

I happened to read the following, but it did not make any sense. "During the runup, when switching magnetos from "BOTH" to "RIGHT", you notice no RPM drop. This indicates the left P-lead is not grounding or the engine has been running only on the right magneto because the left magneto has totally failed" Here's what I don't understand: If the left magneto has totally failed and only the right magneto is being used, wouldn't that instead cause a decrease in RPM? Since dual ignition causes a complete and even combustion of the fuel/air mixture, which results in a higher engine performance and power output.

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3 Answers



  1. Brad on Dec 18, 2014

    By switching from both to right you are grounding and thus shutting off the left mag. You should be on right only and see a slight reduction in power. In your example you see no reduction in power, so with the key set to right you are actually on both and thus not seeing a reduction in power. The left grounding wire is broken:)

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  2. John D Collins on Dec 18, 2014

    While this is a possible cause for the lack of a drop in RPM, it is not the only possibility, as with a badly timed engine, it can also cause there to be no perceivable drop on a Magneto check.

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  3. Kris Kortokrax on Dec 18, 2014

    If the left mag has failed, the plugs are firing on only the right mag when the switch is set to BOTH. When you switch to RIGHT, there is no difference in RPM because you are not cutting out the LEFT mag.

    During the mag check, it will become apparent if the left mag has failed. If you switch to LEFT and the engine quits, then the left mag has failed.

    A further check that I do is just prior to shutdown, bring the throttle to idle, momentarily switch the mag switch to OFF. If the engine keeps running, a P lead is broken or disconnected and should be repaired immediately because the engine could start if the prop is moved after you have shut down by pulling the mixture to idle cutoff.

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