Just found this thread after my search under "bad coil". My 69 440 Charger suddenly took a crap when my wife and I went riding one day. It would barely start and when it did it misfired and you dam near had to floor it to keep it running. Firecore Distributor and coil, MSD 6AL-2. Thought for sure it was the MSD, sent it off to El Paso Texas to have it looked at. Got it back, same dam thing. When I reinstalled the distributor and MSD after having it checked out, it would only start when cold. It wouldn't idle below 38 degrees initial. I checked the MSD and it lit my ass up and dam near blew my fillings out. Then I check spark to the FireCore coil after getting it to run for a little while and had no spark. I then installed my Blaster 2 coil off my Cuda to the Charger and it fires right up, put initial timing down to 18 degrees initial and it runs like it did before. Never have I seen a coil cause this kind of problem, but I'm no expert. Firecore coil only 2.5 years old.

So far I changed the plugs, went with the Autolites that Cab recommended and bought an MSD Blaster coil. Have not started or even driven the thing in several weeks so nothing new to report back yet. The summers here tend to kill my motivation to work on it.

Yep I understand the heat thing, it's 100 degrees here in Texas, I can only imagine how hot Arizona is. Definitely takes the fun out of working on your ride in the heat

the higher the engine load the higher the cylinder pressure. higher cylinder pressures are more difficult for the ignition. inadequacies or failures in the ignition system are more likely to show up at higher loads and engine speeds.

So glad I found the issue. The suspect coil had 32600 ohms between the primary and secondary side. The MSD Blaster coil that I verified the issue with had only 5000 ohms between primary and secondary. Never had a coil go out like this on me before, acting so strange. The faulty coil would only fire when cool and with 38 degrees initial timing. As soon as would try to back timing down below that it was like turning the ignition switch off. It would also quit firing after about 3 minutes of operation. I learned a great deal about ignitions after this fiasco

After changing the plugs to the Autolite 23's, and the coil to a new MSD Blaster, backed the timing off to 18 deg initial I have no more audible ping. Pulls strong through 1st and 2cd gear with no break up. Had to adjust the idle RPM up a bit after the timing change which I expected to have to do.

Today here in Phoenix was a very humid 104 or so. Still have a little bit of a crappy idle condition due to some vapor lock once the engine temp gets to above 200 deg but I blame that on the poor oxygenated pump gas of 91 oct that we are limited to here. Overall happy with the results.

Thanks to all you guys that have helped.