I have a 27 and a 21 t. both run great. however, one has to run on battery and the other will only run on magneto. it is my understanding that the car should run on magneto. so what do I look for on the t that will only run on battery?
First get Ron Patterson's schematic for the cars and make sure everything is wired correctly. For the one that only runs on battery, first see if you have output from the magneto. Get a 12 volt turn signal bulb and socket. Hook the bulb between the magneto post and ground. Using a good analog volt meter read the AC voltage output from the magneto with the engine running. You need to get over 20 VAC I believe at higher RPM. If you have the required voltage, then it is a wiring or ignition switch issue, if you don't have the AC voltage, then magneto work is required.
For the car that only runs on magneto, my guess is the ignition switch or improper wiring. Whatever the case, on the car that runs on magneto, make sure the car 6V battery voltage never gets to the magneto post.
Assuming that the car has a strong enough magneto per John Z's test then if the car runs on battery but not on magneto that generally points to coils not being properly adjusted or having been setup using a buzz box type tester of some sort rather than an HCCT or equal.