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#1 |
Registered User
Join Date: Aug 2014
Location: Frederick md
Posts: 178
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Exhaust backfiring
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
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#2 |
Registered User
Join Date: Dec 2011
Location: mount airy NC
Posts: 308
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Re: Exhaust backfiring
sounds like the timing is off.
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77 c10 (went from a inline 6 to 350) (converted to 3 speed in the floor) 350: mild cam, holley 600cfm 4bbl, summit stage 1 intake, accel billet distributer, flowtech headers, 2 1/4" exhaust with glasspacks, powermaster 160a alternator. |
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#3 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,602
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Re: Exhaust backfiring
Backfire or afterfire?
Backfire is out the carb. Afterfire out the exhaust. And it shouldn't do either. What year? What carb? |
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#4 |
Registered User
Join Date: Aug 2014
Location: Frederick md
Posts: 178
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Re: Exhaust backfiring
79 rodchester.
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#5 |
Registered User
Join Date: Aug 2014
Location: Frederick md
Posts: 178
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Re: Exhaust backfiring
I'm sorry quadrajet
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#6 |
Registered User
Join Date: Aug 2014
Location: Frederick md
Posts: 178
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Re: Exhaust backfiring
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
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#7 |
Registered User
Join Date: Jul 2014
Location: Pacific Grove, CA
Posts: 105
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Re: Exhaust backfiring
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.
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Robert 1975 C10 - Huckleberry 2007 Jeep JK - John Wayne Jeep Smittybilt front/rear bumper, 10K winch Pacific Grove, CA |
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#8 |
Registered User
Join Date: Aug 2014
Location: Frederick md
Posts: 178
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Re: Exhaust backfiring
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
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#9 |
Registered User
![]() Join Date: Aug 2014
Location: Pittsburgh, PA
Posts: 627
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Re: Exhaust backfiring
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.
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#10 |
Registered User
Join Date: Aug 2014
Location: Frederick md
Posts: 178
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Re: Exhaust backfiring
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
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#11 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,602
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Re: Exhaust backfiring
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. |
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#12 |
Registered User
Join Date: Aug 2014
Location: Frederick md
Posts: 178
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Re: Exhaust backfiring
That's nice how would I tell if it's the cam
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#13 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,602
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Re: Exhaust backfiring
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. |
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#14 |
Registered User
Join Date: Aug 2014
Location: Frederick md
Posts: 178
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Re: Exhaust backfiring
If I pull that line what should I be looking for
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#15 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,602
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Re: Exhaust backfiring
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. |
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#16 |
Registered User
Join Date: Aug 2014
Location: Frederick md
Posts: 178
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Re: Exhaust backfiring
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
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#17 |
Registered User
Join Date: Aug 2014
Location: Frederick md
Posts: 178
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Re: Exhaust backfiring
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
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#18 |
Registered User
![]() Join Date: Aug 2014
Location: Pittsburgh, PA
Posts: 627
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Re: Exhaust backfiring
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.
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#19 |
Registered User
Join Date: Aug 2014
Location: Frederick md
Posts: 178
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Re: Exhaust backfiring
So how do you fix that
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#20 |
Registered User
![]() Join Date: Aug 2014
Location: Pittsburgh, PA
Posts: 627
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Re: Exhaust backfiring
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.
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#21 |
Registered User
Join Date: Aug 2014
Location: Frederick md
Posts: 178
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Re: Exhaust backfiring
The motor is bone stock beside headers
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#22 |
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Location: Pittsburgh, PA
Posts: 627
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Re: Exhaust backfiring
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#23 |
Registered User
Join Date: Apr 2014
Location: Bloomington Indiana
Posts: 1,041
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Re: Exhaust backfiring
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.
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Rich Weyand 1978 K10 RCSB DD. |
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#24 |
Registered User
![]() Join Date: Aug 2014
Location: Pittsburgh, PA
Posts: 627
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Re: Exhaust backfiring
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°
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#25 |
Registered User
Join Date: Jul 2011
Location: WV
Posts: 946
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Re: Exhaust backfiring
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.
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72 GMC C2500 402/Th400 first truck (still have) 77 GMC C3500 CC dually Sold ![]() 70 C50 dump truck "Rusty" 87 K10 Suburban Sold 93 K2500 xcab 6.5TD 96 GMC K2500 Suburban 6.5 TD |
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