MTFCA Forum

After 60 years of ownership, the magneto quit. Last started about 6 months ago. Yesterday started on starter then switched to mag. Ran fine on mag. Shut down to adjust bands and add gas. Started up and left house. When I switched to mag the engine died. When I returned, I did the following:
Checked mag output with analog meter - nothing (didn’t put light bulb on it)
Checked that mag wire is switched to coil box when MAG is selected - checks ok
Checked mag post resistance to ground. Best my digital voltmeter tells me it’s 0.7 ohms
Removed mag post. There was one strand of band lining present. Cleaned that off and reinstalled post
Restarted - still dies when switch to mag and no voltage

Guess I should have verified the mag post wasn’t shorted internally while I had it out but forgot. Unless the magnet demons stole my charge, I am stumped. Any others suggestions?

I'm not a mag expert, but I'd think the ohmeter reading is OK. I'd recheck the mag post and make sure it's making good contact with no grounding. Use AC voltmeter setting. Long shot: You may have a break in the winding that opens when the engine is running and closes when it's stopped, or the mag post contact may be somehow affected by heat or oil splash when the engine is running. The ground end of the winding may be intermittent also.

I need to pull the mag post and check it. Will do that tomorrow.

Check the switch also. They do go bad for no apparent reason

Check for excessive crankshaft endplay.

Well, the magneto is now working. The only thing I did was pull the mag post again. Checked it for being shorted - read 30 Meg ohms so not shorted. Disassembled, inspected and found nothing. Cleaned it up, reassembled and rechecked for shorts - checked OK. Re installed and now I have mag voltage again. The output is low 14 volts max but it has always been low. I suspect a the low voltage is a combination of magnet strength, gap and end play. I did not charge the magnets when I put the engine together. As a teenager I had never heard of such. Has anyone used an end play shim on the crank shaft pully? Wonder how long it would be before something wore down?

Make sure the contact point on the mag post is making good contact with small lead contact on top of the mag coil ring. I had a rebuilt mag ring I installed and the engine would occasionally just quit while driving. Switching to battery and continue driving and it was OK.
I decided to look at the mag contact ring point with a mirrow and to my surprise it was off center lining up with the transmission cover!
I wasn’t about to pull the engine to change the mag ring so I took the mag post contact spring and bent it over to one side to have a good contact to the mag ring. All OK now and no more trouble since.
Don’t ask me how I missed it being off center when I put it together! Anyway it’s OK now.

I did take my mirror and inspect the mag coil solder contact. It was centered ok. The only thing I noticed was an indentation where the magneto post had been making contact. Maybe it is now making contact in a better fashion but that didn’t happen after I removed it the first time. Guess we will see how driving goes. At this point it’s still a mystery.

It could still be the switch or a connection. If it quits again, I'd try cycling the switch and see if it helps.

I don't think it was associated with the switch. Sitting in the garage running on battery, there was no voltage on the mag post. If it's the switch, I've got more than one problem.

I measure the end play at less than one millimeter (0.040)