Backfire all of the sudden?
 

The pontiac gods just dont want me to enjoy my car for the last couple months of the season. See other threads for that story.

With that said, the issue du jour - I went to change my distributor cap, rotor and plugs off a perfectly fine running car. Started car up expecting smooth, and got an intermittent backfire coming out the back of the car.

I thought somehow I must have screwed up the distributor install, or its a bad part, so i put the old rotor and cap back on and the backfire is still there.

Not sure what I would have bumped to cause this phenomenon, or if something just coincidentally went bad at the same time i did this cap/rotor change? seems suspect. but stranger things have happened.

Ive got a petronix setup, and it has worked perfectly fine from the get go. the car has a lot of grounds so i never had the issues others have seen. I also have a few month old MSD 6AL that works fine too. And its a tripower.

In this video, you can hear the backfiring in the first 15 seconds or so at start up, then it runs fine, and about 40 seconds in more backfiring out the exhaust. https://youtu.be/4a8KSbzoeqI

How should I trouble shoot this? thanks for any thoughts guys.

Double/triple check plug wires are in prepared firing order

That was definitely my first thought as well. Having done that once, it was similar.

Check the plugs themselves for cracked porcelain, check that the wires are in good condition and seated properly.

Since it's backfiring out of the exhaust, a cylinder not firing could be the culprit, sending unburned air/fuel into the exhaust where heat lights it off.

Sometimes it's a simple problem
 

COUNTER clock-wise rotation. Been there done that.

Too many Chevy trucks.....

And we've had a couple of NEW spark plugs that wouldn't fire - probably dropped by the chimps at Autozone....

Is this a original distributor?

This was the only done to it?
Change plugs one at a time or yank all wires off and take plugs out?

No wire changes, points, coil, etc?

Take the cap off and look inside. See if anything obvious wrong. Rotor not on correctly or broken. Cap carbon button inside still there? Wires not pushed back on far enough?

Then check firing order - 1 8 4 3 6 5 7 2 counter clockwise.

Easy to put a wire back in the next spot over from where it should be.

:confused:

Sounds like #5 and #7 are crossed. Happens all the time.

We just went thru this with Racerboy and his 400 GTO and after 4 days, 128 post and helping him go over everything top to bottom he had got the firing order wrong after clearing up his initial problem!

U guys nailed it. Somehow I butchered the firing order with the first replacement cap, and then duplicated the error when I put old cap back on.

Pulled all wires off, went back to new cap, and re-wired whole thing. Purrs like a kitten now.

Good way to end Friday. It’s miller time.

Congrats!


:)

Easy is best.
 

We've all done this at least once (or twice). Glad it was simple. :beerchug::beerchug::beerchug:

Congrats on getting it back to good!👍👍

Print this off if you like and keep it in the Glove box for the brief times of Brain fog that we all have.

Yeah I don’t know what happened. I def know the firing order, but somehow screwed it up and I did not even consider that. Once again Occam’s razor strikes again.