MTFCA Forum

I have a 1921 roadster, very original, and this morning we went out for a jaunt, I turned the key the wrong way she fired up but was NOT happy. I knew it didn't run good on the mag, but forgot I turned the key the wrong way, after a quick check of everything under the hood, I remembered the key and she was happy as a clam on battery... how come. The mag works good enough for three cylinders but on battery it makes more power and is so smooth. Maybe a weak coil, all comments welcome, thanks Chuck.

Possibly coil adjustment and/or capacitor issues.

If slightly advancing or retarding the spark lever makes it run smoother, your problem could be in the timer or the coil. In this case one segment is not making contact at the same point in the cycle. Or as suggested above, one coil could be out of adjustment. You see, on battery, the spark is constant as long as the timer rotor is in contact with that segment, so a slight difference in timing of one coil is not as noticeable as that of another. But on magneto after the contact is made, the current builds up to the point of sparking. If the rotor passes the contact before the current has time to build up, you will lose your spark. A coil rebuild should also help your car run better on battery.
Norm

Running better on battery than on magneto is a classic sign of coils out of adjustment. I won't get into a discussion about which method is best (Although someone else will likely be along shortly to push one method over another), but coils need to be set up properly using equipment made for the job. Setting them 'by ear' or using a 'buzz box' or just setting the gap ain't gonna cut it on magneto.

At this point, the easiest thing you can do is run a magneto test using a 1156 bulb and a jumper wire.
If the bulb blows, the your mag is running properly and the problem is probably coils, or other possibilities. If the bulb fails to blow, it could be weak magnets, or the 3rd main thrust surface may be worn and it's allowing the crankshaft to move back and forth, which creates too much gap between the flywheel and the mag ring (it decreases the voltage going to the coils). Or, the mag ring may need to be restored.
https://youtu.be/QnMUPNFbVWQ