Magneto troubleshooting help needed

[Eclector]

1923 Depot Hack with a 1926 engine and who knows what else.

• Car starts and runs fine on battery
  Switching to magneto yields unpredictable results. Sometimes the switch from battery is seamless and car runs fine albeit with a slight increase in RPMs. At other times, the switch to magneto power results in marked roughness in the motor. Missing and rough to the point of stalling. 
  Voltage measured at magneto post yields about 8.5 VAC at low idle. Volts increase commensurate with engine speed.
  Cleaned/Tightened all wiring connections at magneto post, terminal block and back of switch.
  Removed and cleaned the magneto post.
  While under magneto power, I have tried wriggling the key switch, wires and connections with no effect.

So as a Model T and magneto newbie, I have some questions:

!. I have not been able to find a good reference regarding the post. Should the contact point be sharp and needle like?

2. Is there any guidance for making sure that everything regarding the post is set up properly? 

3. Assuming that the output of the magneto is good, what are the other areas that could possibly cause these symptoms? 

4. Am I correct in thinking that since everything works fine on battery, the problem is somewhere between the magneto post and the switch?

5. Other than checking voltage output of the magneto, is there anything else I can measure or observe without tearing things apart? 

Any help is much appreciated,

Steve

[namdc3]

Steve,

You will probably get a lot of input on this, but I’ll provide a few first thoughts. This sounds like a switch problem. It’s possible that two connectors are shorting to each other, as it is very close quarters on the backside of the switch. I would look at this very carefully and turn or bend the connectors as needed. It’s also possible there is an intermittent contact problem inside the switch.

1. Yes, fairly sharp. It digs into the solder blob on the field coil.
2. You should feel the point make contact before the post flange seats. You can try fitment without the gasket and should notice that the flange isn’t resting on the hogshead without downward pressure.
3. Many of the things you’ve tried already. It doesn’t sound like you’ve taken the switch apart and inspected the contacts, which may be needed.
4. Probably. Some coils run better on bat than mag if the coils aren’t working correctly.
5. What is the voltage output of the magneto at “full” engine speed?

[John kuehn]

For what it’s worth my 1919 Runabout got to doing something similar a few years ago. First thing I did was to check the mag post. I had installed a rebuilt mag ring a few years before and when installing the mag post I found that somehow the lead contact button was a bit off center from the hole in the transmission cover. What I did was to bend the post coil contact spring a bit to make for a better contact. It was already making contact but being off center must have had an effect in not good continuity.
Haven’t had any trouble since.
If I would have noticed the off center mag contact button earlier I would have sent it back but I didn’t want to pull it all down again to do it!
Hope this can help. Good luck and I bet your problem is something simple.

[Norman Kling]

It could be any of above. Some other things to check. Does your car run smoother on magneto if you either advance or retard the spark a bit? When you run on battery,the spark will come whenever the commutator roller or brush contacts the point, however on magneto, the current begins to build up when the roller makes contact, so if the coils are not adjusted equally, it will be more noticeable on magneto. One coil might need a higher voltage before it sparks. When you are right on the edge of what I call "Node" you will run more roughly than in the center of the node. What I call node is the point where you advance the spark lever the spark suddenly advances. There are either 3 or 4 nodes depending on how you set the timer at full retard.
Norm

[Norman Kling]

Also, if you are running on a 12 volt battery and the mag output is 8.5 V it could also affect how the coils run at idle. Normally the coils should be set to fire at 1.5 amps. They should all be set the same.
Norm

[Steve Jelf]

I agree that the problem could in the switch. The backing plate is made of sort of a thick cardboard that sometimes is warped so that the contacts don't work as they should. If that's the case you can flatten it on coarse sandpaper. You may need to deepen some of the dimples carefully with a drill bit. Sometimes the back plate is held by tabs you have to bend open. You should anneal them (turn them red and let them cool) before you bend them closed to reassemble the switch.

This back plate is held by pins, not tabs. There are slight differences among switches because Ford bought them from different suppliers.

[Bill Robinson]

If it were mine, the first thing I'd check would be the magneto itself. I'd want to know whether I have a strong or a weak magneto. Otherwise, if the magneto is weak, you can spend lots of hours testing many components, and simply wasting your time.
The test is simple and easy. https://youtu.be/QnMUPNFbVWQ

[Novice]

Several members mentioned checking the the switch. some haft to be jiggled to find the sweet spot. Vendors sell a under dash magnetometer / AC Voltmeter. that would allow You to keep tabs on Mag output as You start and drive.

[Eclector]

Also, if you are running on a 12 volt battery and the mag output is 8.5 V it could also affect how the coils run at idle. Normally the coils should be set to fire at 1.5 amps. They should all be set the same.
Norm

Norm, this is a stock 6 volt system

[TWrenn]

And if it's a repop switch, like Steve Jelf said, the problem could be IN the switch. Trust me, the repops are not very well built.

[Steve Jelf]

The switch doesn't have to be a repop to be problematic. An original can have a warped back plate.

[namdc3]

Steve, have you had any time to make progress on this? Any luck?