For the last few months, I've been having problems with my ignition switch or magneto. When I start the car on battery and then switch it to magneto, it usually shuts off immediately. If I switch it back to battery and hit the starter, it fires right up again, but fails when I switch it to mag. This shut off occurs most of the time, but occasionally it keeps running when I switch to mag.
Today, I drove it on battery since it kept shutting off when I'd switch to mag. After fiddling with the wires behind the ignition switch, it ran on mag. I ran it that way for about 2 miles and then it just shut off.
I've disassembled the ignition switch, lightly sanded the contact points, gently lifted the contacts, removed each of the wires on the back of the ignition switch and lightly sanded each to ensure good continuity, and tightened everything.
After spending a considerable amount of time fiddling with things today, I realized the nut on the top of the mag post was loose and the wire was making less than great contact. I tightened it and it worked great when I switched it to mag. I let it run for about 4 minutes - just enough time to put a couple of my tools away - then it mysteriously cut off as I was walking back to the car. It started easily on battery, but will not run on mag.
Any thoughts on what could be causing this intermittent problem? Is it likely to be an ignition switch, a wire, the mag post, or the mag?
Band "lint" on the mag post. Excessive end play in the crankshaft. Loose timer. Bad switch. And so on, and so on and...
I wondered about the possibility of band lint. I will check that out tomorrow.
Without knowing much about the history of your engine and magneto, That's where I'd start.
While it is running on battery attach a 12 volt light bulb to the mag post and see if it remains bright and steady, this will at least tell you the condition of the magneto, it is a start.
And a volt/ohm meter. If you're going to use a volt/ohm meter, make sure it's not a digital. An analog meter should work fine.
Well, the problem is solved, I think. Thanks to the great advice above. I checked the mag post and it was super clean - no sign of lint or other debris. I kept fiddling with the wires trying to find an indication of a short, but no luck (I guess that is actually a good thing). I traced the mag wire to the wiring terminal strip and, just for kicks, decided to unscrew it and inspect the mag wire from the ignition switch and the one that runs to the mag post. The wire ends/connectors were not obviously corroded, but I hit them with a bit of sand paper since I'd done that with everything else and they started getting a good shine to them. Apparently, they were corroded a bit. I connected the two mag wires at the terminal strip, started the car, flipped to mag expecting it to shut off, and it kept running nice and strong. I turned it off, repeated, and achieved the same success. I ran it several times today and kept getting solid performance every time I switched to mag.
So, I think the problem is fixed and we just had a connection at the terminal strip that was failing.
Thanks again for the tips above.