Exhaust backfiring

[Mv7fd]

I took my truck to a shop to get the carb rebuilt and timing redone they also did some egr work and vacuum line work and they call me and say due to the exhaust it'll still back fire some. Is this possible it's a stock 350 with long tube headers that run into flow master 40 series and come out stock location the exhaust looks stock besides the pipe diameter is bigger and it's a true dual exhaust with headers

[77chevyinline6]

sounds like the timing is off.

[geezer#99]

Backfire or afterfire?
Backfire is out the carb.
Afterfire out the exhaust.
And it shouldn't do either.
What year?
What carb?

[Mv7fd]

79 rodchester.

[Mv7fd]

I'm sorry quadrajet

[Mv7fd]

Before I took it in it would only pop and crab when you'd downshift and let off or when you were cruising at highway speeds idk what it does not haven't picked it up yet but the lady said it'll still backfire some due to the exhaust

[johnwaynejeep]

Sometimes mechanics will tell you to expect certain noises that we know are normal, like popping/dieseling due to downshifting a manual tranny. We know these things are normal, someone who is not used to driving a carbed vehicle may not be aware of this and might see it as a problem and demand the mechanic fix it. So this is a cover all.

that being said, take it for a test drive with the mechanic and make sure everything works/sounds the way you expect it too. It's easier to ask questions and point out problems when the mechanic is there with you and before you've paid for and accepted the work.

[Mv7fd]

Well I picked it up and everything is perfect. The way it was explained on the phone was totally different then the mechanic explained it and no backfiring or not normal noise

[dfrank]

I had an exhaust leak in my headers and when you let off the gas at wot it would shoot a 4 foot flame out the exhaust with a loud bang. Was actually pretty cool.

[Mv7fd]

It does have a slight pop out the exhaust when your cruising sometimes at like 55 or 60 but I can live with that i'll just give it a little more gas

[geezer#99]

If you have a slight pop then you might have a leak in your exhaust or a cam lobe going flat.
Figure out what it is now or spend some bigger bucks later.

[Mv7fd]

That's nice how would I tell if it's the cam

[geezer#99]

Pop the valve covers off and watch the rockers. Look for one that's not moving as much as the others.
Before you do that plug off the vac hose to the distributor to see if it's timing related.

[Mv7fd]

If I pull that line what should I be looking for

[geezer#99]

Test drive it like that. See if it stops making the popping noise. Could be a situation of too much vac advance at that speed.
If it doesn't do it, plug it back in and test again.
A process of elimination.

[Mv7fd]

I'll do that. I can get it to pop if I put it in second and drive and get it up to the rpms where I need to shift.but won't pop when I downshift from 4th to second even at a higher rpm but if I get back on the gas just barely at the higher rpm when I downshift it'll pop but when I apply a little more pedal it'll stop popping

[Mv7fd]

I plugged the vacuum advance and went for a drive seemed under powered and still had backfire but it was worse and a little more plugged it back in and had power again

[dfrank]

Originally Posted by Mv7fd

I plugged the vacuum advance and went for a drive seemed under powered and still had backfire but it was worse and a little more plugged it back in and had power again

Something tells me you initall timing and total mechanical timing is off if you lost power with no vacuum advance. I run a 383 stroker with msd 6al-2 and pro billet distributor with no vacuum advance. The vacuum advance is more for fuel economy. I dont know your motor so I couldn't tell you what gour inital timing should be (I need cam specs). For a small block chevy no more than 36 total mechanical advance (with no vacuum advance) or you risk detonation at high rpm. You want the total mechanical timing all in by 2500-3000 rpm.

[Mv7fd]

So how do you fix that

[dfrank]

Originally Posted by Mv7fd

So how do you fix that

Get a timing light, disconnect vacuum advance and plug it, then increase engine rpm till it stops advancing (once it stops then that is you total timing) set it to 36° advanced btdc. Dont go higher than 36 for a sbc. Then let engine idle and read the timing and that is your inital timing. I belive a stock 350 should run around 10-12° btdc inital timing How is the motor built? I am running 18° inital and 36° total with a cam with 214° intake and 224° exhaust duration at .050" lift. If your inital timing reads 5° btdc then your mechanical advance is increasing to much. In a msd distributor there are bushings you insert to decreas or increase the mechanical advance. I don't know if you can limit the amount of mechanical in an hei which is why we always run msd igniton. There are also springs in both hei and msd distributors which control how fast the mechanical timing advances. In a sbc you want it all in by 2500-300 rpm. If this is just a stock motor than the hei should be set right so you can just set it at 36° total and then your intial should fall around 10°. I really need to know how the motor is built to give you accurate numbers for where the inital should be.

[Mv7fd]

The motor is bone stock beside headers

[dfrank]

Originally Posted by Mv7fd

The motor is bone stock beside headers

Then you should be fine just setting total timing to 36°. Just check and make sure the I inital isn't way too low or way too high.

[rich weyand]

It sounds retarded to me if you disconnected the vacuum advance and it got worse. BTDT.

I am running 17 BTDC initial, and 37 BTDC total without problems with a torquer cam.

[dfrank]

Originally Posted by rich weyand

It sounds retarded to me if you disconnected the vacuum advance and it got worse. BTDT.

I am running 17 BTDC initial, and 37 BTDC total without problems with a torquer cam.

I don't think 1° over 36 is going to hurt anything, once you get into the 40's then you are asking for trouble. It is just safer to stay around 36°

[mrolds88]

If you are getting some popping at cruise, it sounds like a lean misfire. Your Qjet should still have the openly adjustable bleed screws on the front. They are on the base casting. angled out slightly. I would adjust them both out about 1/8th of a turn to see what that does. And check your plugs. Color and gap.