need help tracking down misfire

when I start it up and it's sitting at 1700-2000 rpm warming up there is a very noticable random popping in both sides of the exhaust. Once it has warmed up and idles around 700-800 rpm the sound is not noticable, but I can still feel inconsistant "puffs" when I cup my hand over the tail pipes. And even after warmup it's still popping at higher rpm. It's most prevalent when held at higher rpm (~3500) for a few seconds and then let off completely. It crackles and pops loudly until the engine slows down.

It seems that the popping disappears when I pull the plug on the aft cylinders on each bank (#7 and #8). There's no crossover in the exhaust to it's pretty easy to tell which side is still missing and which is not. So then I switched the #8 and #6 plug wires... same thing. So it doesn't seem to be a plug wire issue (although I still intend to get some better seperators and keep them off the manifolds).

I just rebuilt the distributor and used Napa cap, rotor, points, and condensor. The cap looks fine, but that's not to guarantee anything. What else could cause a misfire besides ignition? I think I'll pick up a cheap cap from Autozone and see if that eliminates it, but beyond that I'm stumped...

Any thoughts?

Headers?? as posted in the ignition thread....Jacobs ceramic boot ends and then Techron for the injectors finally smoothed out this engine....

:gurney::clobbered: and that intermittent misfire was ragged as hell, one day the thing was fine, the next had more misses than a HS prom....

:ill::clobbered::gurney: it was kicking my ass pretty badly for several months there....

Gene he's running a carb, so Techron will have little, if any effect. That stuff does work well.

Jason, have you tried spraying water on the secondary ignition? If there's a wire breaking down the water will find it.
Have you had the heads off? I've seen a few that the valves and seats were "iffy" and had a bit of a random miss. High RPM miss I'd think maybe valve springs, but you haven't said anything about that.

Popping out the exhaust...running too lean?

after some more tinkering and talking to my neighbor I've decided to rebuild the carb and adjust the valves before chasing it around any more. A new distributor cap didn't do anything, and I know the carb could use some attention. If I had some oil deflectors I wouldn't have put off valve adjustment for so long. Any best practices for keeping the oil from splattering everything under the hood?

Old towels- shop towels work pretty well, but anything will work. Do one side at a time. Don't park it over anything that will stain- park in the grass if you can.
I have to say this-- it's a pretty longshot that the carb will be causing your miss. A fresh rebuild can't hurt, but it's still a long reach.
FWIW- I think Bird has a set of old covers he drilled to adjust valves. Past that, if you pack rags along the lower side of the head and then lay a rag over the rockers it should keep the oil close to where it's supposed to be.

yep, just talked to Jeff on the phone. He's got a set of covers with holes drilled in the middle. I might see if I can find a set of crappy sbc covers for cheap and drill 'em out.

You're right on about the carb, I don't think it's likely to make a difference in the misfire, but it's been on my to-do list for too long as it is and it certaily won't hurt.