Here is a mystery for the forum to chew on.
I rebuilt my transmission and magneto about a year and a half ago. The car ran like Henry made it until about a week ago. I was pulling over to wait for a buddy who was caught at a light, and when I throttled down the car chugged like it was suffering fuel exhaustion and quit. Knowing I had a full tank and figuring it was just a temporary problem, I started it up again on battery and it ran fine. Switched to mag and it sounded like I had 2 cylinders missing. Back to battery and on down the road. About 2 miles later, I switched to mag and it ran again like new. Figured there was a lint problem and parked the car. The next morning I started it up on battery, switched to mag and had nothing. Pulled the mag post and it was clean. Figured it must be the switch. While the switch tested good, I borrowed a known good one from a club member and installed it, plus a new mag post, and the results were the same. It would run ragged or not at all. And the problem was that intermittent, sometimes it ran badly and sometimes not at all.
I replaced the wires to no avail. I replaced the mag post, again to no avail.
Finally decided that the problem had to be in the mag coils, so I pulled the engine expecting to find a piece of cotter pin or some other obvious problem. The magnets were still gapped just as I had them originally, showing no sign of shifting and the engine has no 4th main or other crankshaft problem.
Found a small piece of shim in the transmission filter, but after taking the mag down found absolutely nothing. Tested the coil and it is strong and lacks any kind of short. Tested the soldered contact point and it is normal.
In short, whatever gremlin was affecting the mag is not at all apparent.
Since the car runs well on battery, that tends to eliminate the coils, (Patterson rebults) and timer (Anderson).
So, if it is not the switch, the wires, the mag post or the mag coils and the magnets are strong enough to operate properly, I wondered if anyone else had suffered this most frustrating situation before I begin to put it all back together and just hope that whatever it was has gone away. Prior that that stop, there had been no sign of magneto problems. In fact, the car would start cold on mag.

I question the statement "Since the car runs well on battery, that tends to eliminate the coils". You need to get the coils tested. Bad coils often function better on battery. Rebuilt by the Coilman or not that is likely the problem.

I should have added that before pulling the engine I tried a total of three sets of good coils with no change in the situation

Bill:

The problem with INTERMITTANT problems is just THAT - they are intermittant. When you HAVE the problem you need to be scientific and try to at least split the circuit so that you can eliminate at least one part of the thing as being the problem. Purchase an ANALOG voltmeter and 1156 bulb and when the problem is THERE - run the magneto test that has been posted here countless times. With intermittant problems that are stubborn, it sometimes is BAD to start swapping things since in fact you DON'T want the problem to go away until you know for sure what is causing it. I never figure I have an intermittant problem licked until I can SEE what is causing it or can make the problem COME AND GO under my complete control. One must be very observant and NOT pull wires and yank on things since if the problem goes away mysteriously - you get to do it all over again when it comes back. When the car is running OK - do the magneto voltage test using the 1156 bulb so you KNOW what it measures when the thing is running OK - then wait for the problem to resurface and then CAREFULLY run the test again to see if you get different magneto readings. Test it several times to make sure you know then whether the magneto is REALLY the source of the problem. DO NOT ASSUME it to be bad or you may get to chase the elusive wild goose for many days. Make all magneto tests AT THE MAG POST so that you aren't fooled by something as simple as a magneto wire connector terminal that has lost its crimp. ABOVE ALL - move slowly and be VERY observant of all connections and if you DO move something to see if it is loose - try to move only one end of a wire rather than just yanking on things.

I will say the poor fellow wasnt in a place to be "scientific" the first couple times it gave trouble.4 lane hwys aint the place to break down a problem.:>)
I witnessed the bulb test and it was putting out like it worked at Maybells on half price day.
That car has allways ran like a sewing machine till that 1 day and it backfired and I just knew I was the wrong section of town.

Do what John says and carry the bulb and voltmeter in the car with you. If the mag test shows the output is good even when the motor is acting up, take a good look at your ignition switch.

Bill,

Sounds like you've looked at everything but the coil box itself. Since the coils tend to fire "hotter" on mag, maybe your coil box insulation is breaking down, (carbon tracks in the wood for instance), and it's only a problem when running on mag. Also, the coil box terminal for the mag power is a different one than for battery power. Could there be trouble at the coil box mag terminal?

Sure is easier than pulling the engine just to check it out.

The car is a 23 fordor so the box is metal. I took the box apart and gave it a thorough check and cleaning before pulling everything. It tested fine. Believe me, the last thing I wanted to do was pull the engine. Worked very slowly downward, even pulled the hogshead to see if there was something visible at the top end.

Okay, my thoughts.
1. Mag output to coil wire
2. Coil box shorting
3. Broken/cold solder joint
4. Cracked insulator

You did say you replaced wires, but that one? The coil boxes seem to be a royal PIA. Water getting into them really sets them off for days on end in my experience. Broken/ cold solder joint on coil winding to the next coil on the magneto. It may be just fine and then go open after a jar, then back to good contact again. A cracked insulator will go to ground sometimes then other times not. Other than that I do not have a clue.

can a cutout sticking cause a coil problem?

Hi Bill,
Had a problem awhile back that sounds similar to yours. Had a wooden terminal junction block screwed to the firewall, to which all the various wires came to meet. Turned out that over the years, the wood dried, shrank, and the wire attachment screws bottomed out against the firewall - as luck would have it, it was the magneto wire screw that bottomed out, rendering the mag output useless. Check your terminal block.
Another time, had a VERY frustrating time with a newly professionally rebuilt mag. The soldered wire attachment screw on the mag ring was screwed in just a tad too tightly - when cold, all was well ... when the metal warmed up, and expanded against the screw, things "grounded out" and failed. Had to undo the solder joint and loosen screw a tad, and resolder. Obviously, I hope that your problem is readily solved at the terminal block. Both problems were quite frustrating, as they were intermittent! Good Luck Dave

Dave, those are both interesting thoughts. It will be easy enough to check the first, but how did you discover the second idea ??
Mark,,,I have a regulator mounted instead of the cut out.
Thanks

Bill,
Cracker-jack mechanic friend installed the newly rebuilt mag, and could not keep the engine running beyond 10 minutes or so (while on mag). Fella is a rather stubborn New Englander, and has been working on cars (mostly older ones) for 50 + years that I know, and was stumped.....what with a "good" mag, and other new parts - and desperately wanted to discover the problem. Took the engine out again, and checked and poked around the mag until he found the problem. Dave

Are you running regular motor oil and not detergent stuff? had a very simular problem after i had eliminated all the easy stuf It was chunks of carbon comming off the undersides of pistons and floating around in the lube oil and shorting the mag coils it was the carbon forming a resistor you have missed something somewhere HAve you checked the wire from the mag post to the ign sw?

Your problem sound familiar. Once, while running on MAG, my '26 coupe just quit, after bouncing over a railroad track, but would crank and run on BATT. Luckily, the first thing I checked was the problem. Turns out, the vibration caused the mag connection point (that screws into the top of the hogshead) to pop off the mag contact inside the engine. I simply unscrewed it filed a new point on the mag point, adjusted it until it pointed straight down and carefully screwed it back in, making sure the point imbedded itself firmly in the center of the soft mag contact solder. It cranked on the first quarter crank and has run like a top since. If yours runs somewhat, it may mean that the mag point is making slight contact, but the vibration of the running engine is causing it to make intermitent, instead of full contact. Good luck. Jim

Bill,
Curious to know if you've had time to discover the cause of your problem, and if so, what was the problem ? Thanks, Dave

Yea,I have been wondering to.

We replace the contact screws on all the mags' contact blocks. We used to use the original length of 3/8", but modern screws' tolerances are such that a very few of them were a few thousandths longer, and Dave's situation could occur. Of course we have always checked for a ground before releasing the new field coil, but unless the field coil is hot, as Dave pointed out, you wouldn't catch it. In all field coils, the steel rivet passes through the contact block quite close to the end of the screw, so just a couple thousandths can make the difference. Since our new contact blocks are made from a very hard, fiberglass plastic, we now use 1/4" long screws (the shorter length in the hard plastic actually holds better than the longer one in the original, softer fibre block).

I got it all back together today, and it runs like Henry made it. I even hand cranked it on mag and it worked. While I didn't find "the problem" it is obvious to me that some small piece of something was washing around and shorting the coils. It finally became lodged somewhere and virtually shut me down. Whether it washed out when I drained the oil, or I blew it out with the compressor I will never know since whatever it was had vanished. But I am confident that was the problem, particularly after I tested each coil and found no grounds. Also tested the contact area to no avail.
While I had it down, I checked all the clearances, put on the new 4th main, and upgraded to a 4-dip pan (what took Henry so long?). I recharged the magnets and flushed the radiator. So, an altogether good session.
To those who offered suggestions, thanks a lot. You came up with some ideas I would never have thought of and I appreciate it.

Bill

Reminds me of a line from a high school play so many years ago -"Ah, sweet mysteries of life!"
Glad to hear that things are working again. Dave

Ah...Mr. Graydon I presume. Or Nelson Eddy.

Fellow high school student played Nelson's part - Dave