The 383 engine in my TBucket had an issue that caused it to sputter and backfire whenever I punched it. I tweaked the timing and carb adjustments and had it running flawlessly. Now it's back and worse than ever. At idle it runs rough and only gets worse when I'm driving it. It acts as if the plug wires are cross-talking, or a cracked cap, bur neither of those are happening. One noteworthy (I think) symptom is that the left bank is running much richer than the right as you can see in the photo.
Any thoughts?
Oh, and before you ask - Holley 650 dbl pumper, choke disabled, Petronix billet distributor and Flamethrower coil, Vacuum advance disabled, centrifugal advance springs in place.

 

Try a power valve

 

Depending on how the old plugs came out..... I would say that looks more like condensation soot? Just a cold start build up of light unburned fuel...
Check vacuum leaks and also... As Iowan said, Power Valve could cause over enrichment.. Also Check the Float Adjustment...

 

Thanks, everybody. I found my problem. I cleaned all 8 plugs, which were completely fouled and black, then I re-checked the idle adjustments and found that both of the driver's sides were opened about a half turn more than the passenger side. I set them all to 1 turn and it's running beautifully again.
I have a new question, but I'll start that as a new thread.
Thanks again!
John J.