PDA

View Full Version : need help tracking down misfire


lsejlowe
10-04-2008, 10:40 PM
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?

mrvette
10-04-2008, 11:30 PM
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....

TimAT
10-05-2008, 03:45 PM
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.

SmokinBBC
10-05-2008, 04:29 PM
Popping out the exhaust...running too lean?

lsejlowe
10-05-2008, 05:08 PM
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?

TimAT
10-05-2008, 05:25 PM
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.

lsejlowe
10-05-2008, 06:08 PM
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.