The Stovebolt Site Forums The Shop Area Engine Shop backfiring mystery

I dont think it's electrical so I put it here.
I checked everything that I know about that could cause backfiring--timing, put in new plug wires and plugs, gas squirts into carb. ok. inside of distributer looks ok, higher octaine gas, adjusted carb. gas/air flow. and now it's started to "buck" when I hit gas to go uphill-and an occasionl "sneeze" from the engine area. Drivers behind me stay way back--takes care of tailgaters. What else is there to check? I've looked at previous posts on this problem. Also tightened bolts on head and carb.
What could it be? :confused:

Just a thought...if you add a little bit more info.....did this condition just start on a good running engine or is this an initial crank on a resto project? What size engine are you talking about? Type of carburetor and anything else you know about the engine.

There are a bunch of really knowledgeable folks on this site, that are sharp on solving problems. The more specific info, in addition to what you've done to cure the problem, will give them a better picture and your problem will be history.

The movement of the distributor housing from the action of the vacuum advance can break strands in the wire from the negative coil terminal to the distributor, causing the coil primary circuit to open up intermittently. The result is like switching the ignition off momentarily, then back on. It will either cause a backfire out the exhaust, or occasionally a pop back through the carb.

Disconnect the wire from the distributor side of the coil, connect an ohmmeter to both ends, and stretch/twist it. See if the wire goes open circuit with the right amount of flex. It's a problem that can go unnoticed, and cause you a lot of grief.
Jerry

I had a bent/broken pushrod on mine cause it to backfire through the intake.

Engine is a 350 with 4bbl edelbrock carb. headers and intake manifold. It was running fine and just started backfiring at 40mph. It got worse as I tried differant ignition timing even when I put it back at 13degrees btdc. where it had been. Now it backfires when I start it and at low speeds especially on excelleration. I took off valve covers and everything looked and sounded good.
I fixed bad connections at the coil but not at the other ends of wires. The vacuum advance may not be working because when I pull hose to carb. nothing changes. Because it got gradually worse might be a clue for someone more knowlegable than this trial and error restorer.

HotRodLincoln...most folks have never heard of this little glitch.....but it also holds true for the Electronic Ignition pickups. If you've got an HEI that is giving you the above mentioned symptoms, broken wires inside the plastic coating right where it goes into the pickup body may be the culprit. A bit harder to detect but just goes to show that the "newer" technology isn't as bulletproof as we'd like to believe.

Dave

Hy guys ,could there be a carbon track in the distributor cap? How long has this tuneup run?

Nope, no carbon track.

If it's an HEI system, also look for a burned hole where the coil mounting bolts attach the coil to the distributor cap. They originate from the underside of the cap, and burn through to the end of the screw, causing an intermittent miss on acceleration. It's much more common on 3.8 L V-6's, but I've seen it occasionally on V-8's.

Jerry

Backfire thru exhaust or thru carb?

Not a HEI system but that's somthing to look into sometime in the future.
Backfire is from exhaust.

Is this backfire like a gunshot or more rumbling without the distinct bang?

If it's like a gunshot, it is probably a valve and/or poor ignition on one cylinder.