Having issues with my 71 GTX, 440-6 4 speed. Car was running fine on a recent drive. On the way home it started missing and proceeded to get much worse. It got so bad I decided to just park it and get it towed home. I figured it ate the points so I put a new set in and it wouldn't start. After messing around with it I discovered no spark coming from the coil. I then installed a new coil but lo and behold, still no spark. I started going through a diagnostic evaluation (instead of just throwing parts at it) and I really couldn't narrow it down. Points are good, condenser checked out, rotor checked out. Voltage at the coil was 11.8 with key in run position, 9.6 or so in the start position. Without really fixing anything car then started and ran fine. I then decided to drive it around and it ran fine for about 5 minutes and then started missing severely. Car died a couple of times trying to get it home and then when I limped it onto my street, it smoothed out and started running o.k. again. I got home and did the only thing I could think of. I got a cold beer and just looked at the car for awhile. I started thinking that something must be getting hot?? and shorting out in the ignition system. I have many new ballast resistors in stock so I placed a new ballast on the car. I checked ohms resistance on the new ballast before installing and it was .6-.7 ohms. It started right up and was running fine in the garage. I was under the hood just looking and noticed that after running for a minute or so the new ballast started smoking. I shut the car off and there it sits. I then finished my beer.
I'm thinking the problem is in the ignition run circuit (the Blue wire that comes out of the ignition switch and ultimately is on the run side of the ballast and is also one of the field wires off the back of the alternator).