Bench testing a Magneto

Thanks paul f

Something weird that I learned about the Case 4JMA magnetos that's different from other magnetos I've worked on- you need to have a wire hooked to all 4 connectors and three of them grounded at a time when you are testing the mag.

I was trying to set the mag to #1 cylinder- and I kept getting a spark out of the #1 lead... 3 out of 4 turns of the impulse.

Robert explained how since the little mags didn't have a safety gap in them to discharge the built up voltage safely, the juice from the other brushes was leaking across and coming out the lead I was testing. Needless to say- I was confused at the time.

I'm sure Robert and Earl Seamands can give you more bench testing info than I can- but I've included a link to one of Robert's previous posts that is helpful.

-Schotte

• Archive post from Robert Kelley - magneto info

When the mag on the C failed at Dublin recently, during testing it was doing much the same thing as you describe. Upon disassembly I found it had a carbon track around the rotor plate causing it to try to fire multiple holes at once resulting in a spark that was too week to fire any cylinder. I cleaned the carbon track, reassembled and had individual hot sparks again.