1961 Chevy Bel Air cranks, but only starts when key is rel

i had a pertronix before and had trouble with it when 12 volts was applied to as they are not designed to run on 12. If you have a blalst resistor, that cuts it down to about 9 volts. But oes this have the stock wiring in it as there should be a resistor wire in the harness to cut it down to 9 volts. So check your ballast on one side with the key on to see what you voltage is. then the other side and it shold be down to 9 volts. If you are using the stock coil when craniking it gives 12 volt then when you go to run it drops to 9 or even lower. The pertronix igition is a hall effect switch. you might ocntact them via email or talk to them and they may be able to tell you what is wrong. As long as there is powergoing through the resistor all the time you might try taking th e12volt wire off the coil if it's there. See if that cures your problem.

The switch in that car often gives trouble, have replaced lots of them, doing the same thing. It can be checked. The best thing to do is pull the switch out or the dash, not to hard to do on that car, find the contacts between power in and ing on "run" using continuity meter, then turn switch over to start and see if it holds, turn it a couple times, might just have a dead spot in switch.