Working on a 1922 Center Door. It fires right up while on battery. If the throttle is not advanced/opened sufficiently when we switch to magneto, it dies. If the throttle is pulled down far enough, we get enough output from the mag to keep it running when we switch from battery to magneto. The test light shows a dimming of the bulb when the throttle is reduced and an increased intensity when throttle is opened.
Why is the mag output varying so dramatically that it can’t keep the car running at idle?
The mag always puts out more with the engine running faster. What you have is a weak magneto. This is usually caused by one of two things. Either too much endplay in the crankshaft which pushes the magnets farther from the windings, or the magnets have lost some of their charge. There are many posts on how to check for endplay or how to charge the magnets in the car. If the endplay is OK you can try to recharge in the car, but if the endplay is great, you will need to address that before doing anymore. One temporary solution would be to run on battery until such time as you want to do other engine or transmission work which would require pulling the engine. Endplay should be .004 inch on a newly rebuilt engine. Endplay of .015 would be the maximum before you have magneto problems.
To check endplay, push down the low pedal all the way and release. Then go to the front of the engine and measure the distance between the lower pulley and the camshaft cover. Then pry gently with a screwdriver to move the pulley forward as far as you can without bending anything. Measure again. The difference would be the end play.
The club publishes a very good book "Electrical System" which explains step by step what is needed to recharge the magnets and much other good information about the magneto
Crankshaft end play can be checked at the crankshaft pulley by using a long screwdriver placed between the block and back side of the pulley. As the pulley moves back and forth the gap at the mag ring changes, which, in turn, causes the voltage to vary. The excessive end play is usually caused by excessive wear of the thrust surface of the 3rd main.
A way to get a idea about the possibility of end play being the major problem;
Jack up the rear axle and place on GOOD jack stands.
Start the car and put it in high gear at idle.
Does the ignition perform better?
Here is the logic;
When the clutch is in neutral the crankshaft is being pulled back and aggravating the end play situation. In high gear the crank is pulled forward by the magnets.
If there is no discernible difference then a simple in car magnet recharge will very likely solve your problem. This “fix” has been done thousands of times since the T was created. Plenty of information available online!!
You can also use a shim behind the crank pulley to close the end play gap if that is the problem. I put one on a very tired engine that would not run on mag and also charged the magnets. The results were amazing.