MTFCA Forum

I took a 35-mile drive today and the last half a mile was up a steep, rutted and rocky road when my 26 Coupe quit. Flipping the key over to the battery it started immediately, but when switched back to mag it quit again. I managed to get to our destination on battery and when starting for home after lunch it was the same scenario. However, down the road a mile or so I flipped back to the mag and it ran good, but in another mile, it cut out again. I tried switching back-and-forth a few times on the return trip without success, but when I parked in my garage I tried the mag once more and it worked. Then, after simply gunning the engine several times (just to hear how good it sounded!) it cut out again.

The magneto wire is solid at the plug and there are no loose wires behind the switch or poor wires along the way that I can see. There is no unusual noise suggesting a loose object in the transmission. When the magneto works it runs smoothly, as it does on battery, too.

I'm looking for suggestions on how to proceed with a diagnostic protocol and hopefully find the problem before having to tear into the magneto itself.

Often when this happens, your muffler will be left on the road somewhere. If you still have a quiet car, you are ahead of the game!

take a look at this: http://www.mtfca.com/discus/messages/59 ... 1480292299

I think Scott gave you the best advice about repairing your ignition switch and it would not hurt to check your magneto contact as well.

Like David said above, start with the easy stuff first, i.e. the mag post. Remove it and I'd be willing to bet you have all sorts
of lint on it. That'll stop you dead when it gets heavy enough on it. Also, any "foreign metal" from within the trans. that decides
to "float around" may find itself up there and short it out. I've had that happen.

Could be something simple like a corroded switch, or might be something not so simple like a worn rear main bearing allowing fore-aft play in the crankshaft which would allow the magneto gap to change.

Like the others have stated, I'd start by taking the switch out and inspecting the contacts and connections.

The problem may be in the switch. Take the switch apart and check all the contact points. In my expierence the tabs can be loosening the tension against the contacts. Use some fine sandpaper and lightly sand all of the contact areas and carefully bend out the spring tabs for better contact. That usually will take care of the problem.
I would also remove the mag post and clean out any lint or debris if there is any there.
Hope this helps.

I would say he most probable is excessive end play in the crankshaft. In the day there was an accessory sleeve that fit behind the front pully (it was like a brass washer) that took up the end play slack.

I agree with the above. Another thing to question is the output of the magneto. To test you need a 12 volt light bulb to be connected from the magneto plug on the hogshead to ground. Then take an analog AC voltmeter across the light bulb. With the engine running on battery, the light should light up and at slow idle produce at least 6 volts AC. At high speed up to 30 volts AC. If it is less than 6 volts at idle, you have a weak magneto and it won't run very well when the engine is running at slow speed, such as starting out, or after you shift to high, or going uphill. To compound the problem, your flywheel will move as far as possible to the rear of the engine when going uphill. Worn thrust on rear main will increase the gap between the magnets and the coils causing a weak magneto.

My first suspicion would be the switch. Mine did the same thing until I rebuilt the ignition switch and since then have had no magneto problems.
Norm

Thanks, everyone. When I get at it I will start w/ testing the magneto output under various inclinations. That, unfortunately, makes sense as the first incident occurred when climbing up that rocky, rutted >5% road that was bad enough that my door kept unlatching--something that never happened before or since. I have areas on my property I should be able to duplicate those conditions with slopes as steep as 20% under pasture conditions. If that doesn't pan out I'll go to the switch next, although that was recently rebuilt.

I'll post the findings, but we are getting ready to leave on vacation so it may be awhile.