Misfire at cruise, popping/afterfire at 3000RPM

Righty Tighty said
The valve covers have been off and I manually checked the rockers for preload, but I didn't actually measure. I sent the heads in for a valve job, their cylinder head guy said virtually none of the valves were sealing. With the heads (and intake) off, I was able to get eyes on the cam, and all lobes looked good. I also pulled and checked all of the pushrods, none bent.

Heads back on, car runs much better, except the misfire appears now around 3400 instead of 3000RPM.

I forgot to mention that I suspected a lean condition causing the misfire, so I richened the center carb just a bit, which may be the cause of the dark plugs.

Haha, I thought the cure was fuel injection... :D

I will say that the misfire happens even with all new plugs. I'm running Autolite 85 plugs, which are heat range 5.

I'd been following the thread for a while and didn't realize it was a sixpack until yesterday. Have you tried disabling the outer carbs to see if you can isolate it to fuel? Does the misfire only happen under load? Or can you replicate it in the driveway? If you can replicate it in the driveway, I'd hook up your timing light just to verify a spark and go from cylinder to cylinder and also at the coil to verify everything is firing - especially during the pop/backfire.

This is a strange one...