A friend has a 1917 roadster P/U with a flaw. The car was totally restored by grandpa about 30 years ago,complete engine rebuild etc. The car has only been run about a 20 hours since its rebuild.
Now the problem. She runs strong on Mag for about 30 - 40 minutes then stops. Switch it to batt... free start and will run all day. We've sorted out the usual suspects, mag pickup, wiring and switch are perfect. It has four new Ron Patterson coils, new wiring loom, new timer. No visible debris on or around the mag ring contact, and when we changed the oil there was no debris in the old oil. Everything seems perfect. We ruled out fuel problems as it has a new and very clean tank, new fuel line and rebuilt Holly G Carby. Gasoline is fresh.
The engine is in such good tune that every start, even from cold is a free start on Batty. I have started it from cold on mag several times, after choking, two half pulls is all it takes to start up. The symptoms are, once it warms up the mag goes away. Let it sit for about a half hour and mag is back.
The only thing I have not checked is crankshaft end play, but the engine did have all new babbit when rebuilt. The only other unknown is the Mag ring condition, but from the rebuild snapshots it does not look new.
What am I missing?
John if the mag ring seems like it might be having the problem once it gets warmed up maybe something is shorting after it gets warm have you switched it back to mag after the free start and does it shut off or keep running.. From a cold start with an analog meter Check the output voltage at the mag post and see if it starts decreasing as it warms up
Not necessarily directly applicable, but my 1971 Plymouth Barracuda convertible had a similar problem, it would start right up and run for about 20 minutes, then die. Open the hood, let it cool off, it would restart and you could drive it home.
Turned out to be an intermittent open circuit in the electronic distributor pickup coil that only opened up when the coil got hot.
Maybe your friend's T has a similar thing going on in the mag coil?
Fortunately, the car will run quite a while on battery, so the situation doesn't need to keep him from enjoying the car until he's ready to tear into it to replace the mag coil.
I would check the contacts inside the ignition switch and any other connections and contacts in the circuit from the switch to the mag and the coil box. Sounds like you have a bad or loose connection somewhere that is heating up and expanding to the point that it loses contact. After it cools, it makes contact again, similar to the bi-metallic strip in a flasher unit.
I had a modern car in the shop doing this one time. There was a bad connection inside a plastic plug in the wiring harness that would intermittently lose connection and the car would die. By the time a mechanic would arrive to where the dead car was, it would start and run fine, maybe for a few hours, or a few days, and then I would get a call again. It eventually got worse to the point that we could get it to fail in the shop and that is when we found the bad connection.
We checked the switch contacts and they are clean and tight but the coil box wood is bad so we are replacing it with a Fun Projects kit.
After the box is rebuilt and rewired, and if the problem persists, we will measure the mag output at the mag post to see what the output is doing per G.R.'s thinking.
I guess that would leave crankshaft end play or an intermittent open in the mag ring coils..?
The owner is a firefighter so our next work day will depend on his schedule, probably next week. I'll keep you posted.
The problem might be temperature sensitive. I would suspect the magneto plug at the top of the hogs head. As the engine and transmission warm up, that hogs head could expand which would pull the plug farther from the magneto contact inside. Try removing the plug and first check for particles such as band lint on the contact. If you have a coil spring contact, try stretching it and re-install. If you have the magneto oiler type plug, sometimes the spring and plunger do not press hard enough to make a good contact. The best fix for that problem would be to add solder to the contact at the top of the magneto, but that would require removing the hogs head and use of a very big soldering iron. I have successfully fixed this type of problem by installing a washer inside the magneto plug which lowers the contact just enough to make good connection. You will need a fiber washer or some way of keeping the electrode insulated from the body of the plug so it doesn't ground it.
It could also be a loose connection between the plug, the connecting block, and the ignition switch or the switch itself, but those connections do not usually expand with warming of the engine, but they could come intermittently loose with bouncing of the car.
Good luck on finding a fix for your problem.
Thanks Norm, The contact is a brand new oiler type. We will check that again.