Exhaust backfiring

[Mv7fd]

Well I've decided to just let it be it starts good hot or cold and it runs good it only does it when you run the rpm's up high and they never need run up that much

[Desert1957]

Sounds like typical Long tube headers with Flowmaster 40 mufflers.

Everything I have ever head 40's on has had that problem.

Complete exhaust system has to be sealed tight, any leaks let clean air to be sucked in the exhaust stream which can light off causing popping and rumbling.

Desert

[dfrank]

Originally Posted by Desert1957

Sounds like typical Long tube headers with Flowmaster 40 mufflers.

Everything I have ever head 40's on has had that problem.

Complete exhaust system has to be sealed tight, any leaks let clean air to be sucked in the exhaust stream which can light off causing popping and rumbling.

Desert

And that is why over time my exhaust has been welded at every connection except at the header collectors. Hope I never need to change mufflers or take it off. Lol

[Mv7fd]

Wouldn't it be a constant problem then? Because it's only at a certain rpm if held there at a constant

[dfrank]

Originally Posted by Mv7fd

Wouldn't it be a constant problem then? Because it's only at a certain rpm if held there at a constant

I don't think the exhaust is your problem. I think you need to up the jets on the secondaries orif you only have a two barrel then up the jets on them. Like one person said it does sound like a lean condition. It is a very simple fix. What carb is on this? I could probably help you out more on how to change the jets if I know what carb it is.

[Mv7fd]

Stock quadrajet. I found the collector bolts were loose tightened them and it helped

[Mv7fd]

Stock quadrajet. I found the collector bolts loose tightened them up seems to have helped

[dfrank]

Originally Posted by Mv7fd

Stock quadrajet. I found the collector bolts loose tightened them up seems to have helped

I would up secondary jets a few sizes. I haven't done any jet changes in a quadrajet only edeedelbrock's and holley's. Watch some youtube videos on how to change quadrajet jets and that should give you a guide on how to do it. I doubt it is very hard though. Maybe easier than a holley since you might not have to take the carb out and fuel bowl off to change the secondary jets. Good luck.

[Mv7fd]

I'll look into it. I just figure I had the carb rebuilt Monday by a very reputable shop I figure they would have checked that

[dfrank]

Originally Posted by Mv7fd

I'll look into it. I just figure I had the carb rebuilt Monday by a very reputable shop I figure they would have checked that

There is no way for that shop to know what jet sizes are needed for you carb and motor. It is trial and error. You would have to be very lucky if they would of put the correct jet size in for you combination.

[Mv7fd]

Well I replaced the distributor tonight and the pop and crack got worse now it'll do it when you down shift and use the engine to break so now I'm thinking it's the carb

[Your41Plague12]

Originally Posted by Mv7fd

Well I replaced the distributor tonight and the pop and crack got worse now it'll do it when you down shift and use the engine to break so now I'm thinking it's the carb

This makes no sense. If you replaced the distributor and it got worse, it sounds like your issue is timing.

[cadillac_al]

mrolds88 nailed it. The way I understand the description (?) it sounds like typical lean pop on deceleration. I used to like the lean pop in my hot rod Impala but when I changed the engine it went away. I have been meaning to lean out the idle circuit to see if I can bring it back. It sounds tuff when idling down a hill coming into town.

Are you saying it backfires at high rpm's too? Is this just on decel or is it while revving? You can't change secondary jets in a Qjet, you change the needles. All the Edelbrock Qjets used the same secondary needles so it isn't too critical. Around here, people are too cheap to buy needles so they just bend the hanger to raise the needles a little.

[Mv7fd]

When I run it up between 2600 and about 3 grand and hold it it'll pop and crack give it more or less it'll stop. I put the new distributor on last night and now when I down shift and let off the gas it'll pop some

[geezer#99]

What's your initial timing?
Popping like that is from lack of timing.

[Mv7fd]

12

[geezer#99]

What does it do while driving down the road between those rpm's?
What distributor?

[Mv7fd]

Accel and it runs good when your in fourth highway speeds it'll pop some only when it gets up close to those rpms and you give it more or less and it stops

[geezer#99]

Here's a thought!
With the motor off, poke a small pencil into the vent on the carb. The vent is the tube that sits above the choke blade.
You should be able to push and feel the metering rods moving down and back up. They only move about 1/4 inch. You won't hurt anything. It's just the hanger you're poking.

[Mv7fd]

Just adjusted the distributor and got rid of the downshift pop now I'm back to the same old same old problem so my thought is carb problem

[geezer#99]

Adjusted the dizzy to where?

[Mv7fd]

36 degrees over all

[geezer#99]

Tells me nothing. Sorry!!
36 now at what rpm?
What's your initial at now?

[Mv7fd]

36 overall 12 initial

[geezer#99]

You where at 12.
If you set it for 36 over all and moved the dizzy to do that, then you're not at 12 initial anymore.