For the last 10 years I've had an MSD Streetfire box driving leading Diamond coils, stock trailing configuration.
I have FC coils from my first rebuild and swapped them in with a bracket made from aluminum angle. With the leading coil behind the strut tower the trailing coil can go in the stock location so when I put the AC compressor back in it won't be in the way as much.
The install was during the week and I didn't drive the car until last night. Started fine but seemed weak to accelerate, my daily has a V8 so maybe the backside dyno wasn't calibrated.
Coming back after a car wash and about 30 minutes of mostly freeway driving the idle got really low at a traffic light. I pulled into a parking lot to pop the hood, couldn't find anything wrong, but then it dies as I'm trying to back out of the spot.
It wouldn't start until it cooled off for over an hour.

Swapped everything back today and drove the same route in the daytime with bad traffic, ran fine.
The FC coil being bad seems cut and dried, but a couple of things I tried last night should have worked and didn't.


By luck the break down was in a parking lot one block from my place so I was able to grab tools, plugs, and an old coil.
Fuel was flowing to the carb, and the FC coil was the only change so the first thing I tried was connecting the MSD back to the Diamond coils with new plugs, and no trailing since I didn't want to mess with the bracket. No go.
So was the MSD suspect? The next thing I tried was swapping the trailing igniter to the leading side with the old connectors to the coil. My wiring is shamefully shadetree with the old connectors tucked away.
I laid a new spark plug on a strut tower nut to check.
After that didn't work I was resigned to getting help the next day so I wired back the FC coil and everything else the way it was. Nothing to lose, I tried again and there was spark so I was able to drive home. Trailing igniter died on the drive across the street.

To satisfy my curiosity, this evening I tried another spare igniter (got a few back when FBs were more common in scrapyards) on the leading side and got spark with one of the old plugs, so it should have worked last night. I was already tired from a long day, so maybe I didn't connect the right yellow wire?

Thoughts? Everything points to the FC coil but the other fixes should have worked. I guess the only way to be sure is to put it back and hope it fails in a similarly favorable location.

 

seems like the FC coil is suspect, but the MSD box is also a possibility