MTFCA Forum

Hello,
My 26 touring has been losing juice intermittently from the magneto. Sometimes it runs great on the mag and sometimes there is nothing. I am planning on taking the contact out and cleaning it well tomorrow. I reckon that I will also check the wire for any bare spots while I am at it. Just wanted to see if I should be looking at anything else while I am at it. The only other possibly pertinent thing is that the car has an outside oiler that, of course, mounts into the mag post but I don't think that that could have any effect unless the insulator inside has cracked, but that wouldn't be an intermittent problem. My generator is in need of rebuilding at the moment so I kinda need the mag to be reliable. On a side note, how long can you run on battery? I have been driving a lot (around town) and haven't charged the battery in weeks and it is still showing just over 6 volts.

As always, thanks and God bless.

Sounds right. Consider the ignition switch also. Clean & check tension on the contacts.

Recent issue, similar behavior, the contact button contacts are held on with 2 rivets inside the ign. switch. One rivet head came off & the contact tabs shifted & internally shorted.
The magneto worked intermittently & weakened the mag by putting dc power into the mag. If not for a power rectifier, the magnets would have been totally discharged.

On some of the magneto plug oilers, the contact is not quite long enough to make a good connection. A few drops of solder on the tip, might help or the plunger can be lowered by placing insulated washers. Be sure this is the case before you do any modifications to it. Another thing which can happen to a magneto is when the magnets are losing their charge, you could get spark at higher speeds but it won't idle on mag. It also has 3 or 4 points in the rotation of the timer where you get the best spark. In those cases a magnet recharge can be helpful.
Norm

Check for excessive crankshaft endplay.

Art Mirtes

In the day there was a brass accessory spacer that fit behind the front pully with two bolts to correct any excessive end play. Excessive end play can cause magneto failure.

Weak magnets will cause poor magneto performance when starting. Many cars that are difficult to start on magneto will run very well on magneto once started. Magnets weaken very slowly, and once weak, they stay weak. An exception to that would be if some electrical problem caused sudden discharge of the magnets. In that case, failure could be sudden. In any case, weak magnets from any cause will not regain their strength without service. A magneto that works sometimes and doesn't work at all at other times has an electrical issue, or there is an issue with wiring or with the ignition switch.

Crankshaft end play repair shim: Lang's Part # 3030shim https://www.modeltford.com/item/3030SHIM.aspx

NOT a recommendation, just an observation, jb

I have checked everything magneto related (even recharged the mag) and it has to be in the switch. How easily serviced are the 26 switches? I would have taken it apart while I had it out but I am always fearful of breaking off those little ears that are bent to hold the back on.

A Jeweler's torch is best for annealing those 4 tabs - a very good replacement back is/was available through Lang's.

Yes, turn those tabs red and let them cool so they will bend back without breaking. Sometimes the problem in the switch is a warped backing plate. Often that's an easy fix that doesn't require replacing anything. You just flatten the plate on coarse sandpaper. If that makes any of the dimples too shallow you can deepen them with a drill bit.

Some switch back plates are held by tabs and others have pins, because they were made by different suppliers. If you do end up having to buy a new backing plate you may need to file a slot or two to make the tabs fit. Again, different suppliers.

RajoRacer wrote: ↑

Wed Feb 08, 2023 2:20 pm

A Jeweler's torch is best for annealing those 4 tabs - a very good replacement back is/was available through Lang's.

If the tabs get broken, very small pan head sheet metal screws can be strategically placed to hold it back together into the edges similar to the "pin" style switch body.

colonelpowers wrote: ↑

Wed Feb 08, 2023 2:16 pm

I have checked everything magneto related (even recharged the mag) and it has to be in the switch. How easily serviced are the 26 switches? I would have taken it apart while I had it out but I am always fearful of breaking off those little ears that are bent to hold the back on.

Have you checked the magneto output voltage? Have you tried running magneto power directly to the coil box, thereby bypassing the switch**?

**If you do that, be super careful to NOT send any battery power into the mag. Remove the battery wire from the coilbox entirely and use only a jumper wire from the mag post to the coilbox.

I haven't tried bypassing the switch to run on just the mag but I believe that it should work fine. At a little above idle speed the magneto is putting out about 20 volts. It could be better but I don't think that it is a bad reading. I may try to charge it again sometime.

If you've measured correctly, 20V a little above idle is excellent

This is where a St. Louis Mag Tester really shines

I wouldn't go to any effort at all to recharge based on that result