I just finished overhauling a Lycoming O-360; it's currently on a stand, i.e. it's not in the aircraft. I replaced the capacitors and point gaps on both the L and R mags. I adjusted the E-gap and engine timing at 25 degrees as per the engine plate.

I have two toggle switches connecting the P-leads to ground: one for the R mag and one for the L mag. I turned on the L mag only to start and when I started the engine for the first time it fired up right away. But when I then turn on the R mag the engine just stops.

I can start the engine on the L mag only any time. As soon as I turn on the R mag, the engine dies. It’s as if when I turn the R mag on the L one goes off. I did everything I could to troubleshoot but I did not find where the is. What could be the issue?

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    $\begingroup$ In many GA aircraft it's common to have a switch "Off - L - R - Both". Setting the switch to R would indeed shut off the left mag. $\endgroup$ Feb 20, 2018 at 18:18
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    $\begingroup$ Hi Dan, this engine is mounted on a stand and what I have is two toggle switches connecting the P-leads to ground One switch for R mag and one for L mag. When I turn the R mag switch on the L is still on. So, that is as if I were turning the regular GA switch to both... $\endgroup$
    – Lewis
    Feb 20, 2018 at 20:35
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    $\begingroup$ @DanPichelman By the content of his question I think it's fair to assume he knows how magnetos work. $\endgroup$
    – BDLPPL
    Feb 21, 2018 at 9:25
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    $\begingroup$ Wild guess (I do not have wiring schematics at hand nor am I sure such situation is actually possible): What about broken grounding of primary coils, but both still connected together? Then, one magneto switch, while still closed, could provide ground reference, but as soon as you open it too, both coils go floating. Btw. does engine run with R mag only (L shorted)? $\endgroup$
    – Martin
    Feb 21, 2018 at 11:22
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    $\begingroup$ Does the engine fire up on the right mag or both mags? $\endgroup$
    – GdD
    Feb 21, 2018 at 13:45

2 Answers 2


There's 3 potential problems. Magnetos are very simple in how they operate, and the system basically runs on 3 things that can be easily tested.

  • Spark plugs, engines can die because of 1 bad spark plug, I've had engines become extremely rough and die within 5-10 seconds when checking R/L because of 1 plug with a piece of lead shorting the electrode.

  • Magneto itself. I highly suggest against opening up and replacing anything in high-time magnetos in the field for multiple reasons. In case of Slicks, the magneto components are designed to wear at an even rate i.e replacing point contacts or a capacitor is not a wise decision for maintenance or for cost-effectiveness over time. An overhauled 4300 series magneto is ~600 dollars, the labor and parts for internal parts reaches that quickly for a substandard magneto as the finished product. I can't speak as much on Bendix but magnetos are a mix of old, complicated, and mechanical that means high wear and high chance for improper reassembly. In this case I assume your O-360 is running 437X Slicks.

  • Wiring in the ignition switch. You cannot improperly wire a magneto(as far as the operation of the magneto is concerned on a 360, any absence of wires won't stop the operation), and personally this is the least likely of the 3 especially if you didn't touch the ignition. Improper wiring of the backend or improper switch installation could cause this, but again I would highly advise checking the 2 before.

I know this is long past a point of help but I think it helps to explain for anyone else interested


If the R magneto was inadvertently providing a very advanced spark the premature ignition would effectively brake the engine very quickly. I'm not familiar with your engine but did have that happen with a classic Kettering (points/capacitor/distributor) ignition in 1977.


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