Not all cylinders are firing when ignition is switched to the magneto.

[George1348]

I recently put a new mag post on my 1926 coupe. It didn't run when switched to the mag before, and because the internal pin on the old post was bent. It runs fine on the battery. So I guess the old post might not have been the problem. However, now it will only kind of run on the mag when the spark is advanced somewhat high. But not all the cylinders are firing when on the mag. My dad described it as "intermittent." What could be the problem here? Could it be something with the coils? They're brand new, purchased from Snyder's shortly after I bought the car back in April. It can't be the spark plugs, because they work fine on the battery. Is it something with the magneto itself?

Ok, people are telling me to check the magneto's output. What is the correct output? What would be considered "low" output?

EDIT/UPDATE: I disconnected the wire from the mag post and tried to test the output with a meter. Unless I have a bad analog meter, there was NO OUTPUT, regardless of how the spark and gas levers were set. I also tested it with a compass by manually turning the engine over. The compass moved when the engine turned over, so there must be some sort of charge in there still.

EDIT/UPDATE 2: I got in contact with the local model t club, the guy who replied to my email said that if it runs fine on the battery but bad on the mag, it sounds like the field coil assembly is dying.

[stevests]

Just because you purchased "new" coils, it's best to have them checked out for proper setting by a competent "coilman" such as Ron Patterson or Brent Mize.

[George1348]

Originally Posted by stevests

Just because you purchased "new" coils, it's best to have them checked out for proper setting by a competent "coilman" such as Ron Patterson or Brent Mize.

I'll contact the local club and see if they have a guy who can check them. And if they don't, I'll find someone to send them to.

[J Franklin]

You can check the mag output with a volt ohm meter. Check on the AC scale.

[George1348]

Originally Posted by J Franklin

You can check the mag output with a volt ohm meter. Check on the AC scale.

Ok. What should the output be?

[rotorwrench]

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Voltage increases with rpm. It's basically an alternator with no diode bridge to DC. It's all AC voltage. The likely problem is in the pick up coils on the coil plate assembly. The magnets may also need a recharge to restore magnetic gauss (residual magnetism). If the trembler coils function OK on the battery then they should work on the magneto as long as it's in good working order.

The old headlight bulbs that worked off the magneto before they started installing starters and generators, were 24-volt bulbs but they could handle voltages up to 28-volts or more as long as it didn't get too high. Most folks drove slower at night so they wouldn't out run the headlights.

[harryc]

make sure the coils are making solid contact in the coilbox. sometimes they will jiggle around and cause a misfire. Also make sure the engine will run smoothly with the battery before concentrating on magneto issues

[George1348]

Originally Posted by harryc

make sure the coils are making solid contact in the coilbox. sometimes they will jiggle around and cause a misfire. Also make sure the engine will run smoothly with the battery before concentrating on magneto issues

It does run smoothly on the battery.

[29spcoupe]

Coils should be set at 1.5 -2 volts when vibrating with spark plug attatched.

[Kelly mt]

As stated check your magneto output voltage. Use a analog meter on AC. If it's low you can charge the magnets in the car. I used the method using a compass and three car batteries in series. It worked OK.

[George1348]

Originally Posted by Kelly mt

As stated check your magneto output voltage. Use a analog meter on AC. If it's low you can charge the magnets in the car. I used the method using a compass and three car batteries in series. It worked OK.

What is considered "low" in this case?

[J Franklin]

Originally Posted by George1348

What is considered "low" in this case?

Give us your readings and we can discuss the outcome.

[George1348]

Originally Posted by J Franklin

Give us your readings and we can discuss the outcome.

As I said in my edit/update to the original post. There is no output at all, but there seems to still be some charge in the magnets, as I checked them with a compass and by manually cranking the engine.

[J Franklin]

If there is really no voltage from the magneto the coils will not buzz or do anything. Are you reading the AC scale?

[George1348]

Originally Posted by J Franklin

If there is really no voltage from the magneto the coils will not buzz or do anything. Are you reading the AC scale?

I am. They buzz when it's set to the battery. I'll test it with a meter again tomorrow.

[J Franklin]

I thought you had said it fired intermittently on mag. If so and now you have no voltage fdrom the mag during the test it may be a problem with the mag post.

[George1348]

Originally Posted by J Franklin

I thought you had said it fired intermittently on mag. If so and now you have no voltage fdrom the mag during the test it may be a problem with the mag post.

It did. My dad and I don't get it either. I will check the mag post tomorrow. It's brand new though, so I can't imagine there's something wrong with it, unless it somehow isn't seated properly.

[29spcoupe]

Sometimes fuzz from the band material will collect on the mag post and prevent current from reaching the magneto wire.