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. $\endgroup$ – Dan Pichelman Feb 20 '18 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 '18 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 '18 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 '18 at 11:22
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    $\begingroup$ Does the engine fire up on the right mag or both mags? $\endgroup$ – GdD Feb 21 '18 at 13:45

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