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#1 |
Registered User
Join Date: Apr 2004
Location: Clanton, AL
Posts: 1,253
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Backfire on deceleration.
I just put a 400sbc in my '70 & a Holley 4175 (650cfm spreadbore) I had sitting around. I am running a HEI ignition system w/ Summit 50k coil. I get a little over 20" of vacuum at warm idle. Cheap headers to dual exhaust.
My problem is that when i let off the gas, is sometimes gives a popping exhaust backfire. This happens more if I'm getting on it & then let off. Any ideas what causes this?
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85' CUCV M1009!!! The newest addition! 6.2diesel, Th400, NP208, & only 36k miles! 70' C-10 LWB Fleetside - Looking good these days! ![]() 05' Dodge Neon 88' Winner Escape Sport 1750 - 4.3v6 94' Seadoo SP 84' Honda TRX200, bare nekkid. Just a frame & tires. Always looking for another project or any good deal! ![]() |
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#2 |
Project underway - finally.
Join Date: Feb 2007
Location: Greenville, SC
Posts: 493
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Re: Backfire on deceleration.
OK, depending on the noise, this may be normal. For a relatively open exhaust, on decel you'll hear "poppity pop pop pop" in a sedued sort of way. If it goes "BANG!" then it's not normal. I am not exactly sure why, but it's a result of scavenging happening backwards or something like that.
Mine does it because the exhaust leaks more than a kitchen strainer. -Brian
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1972 GMC Pre-resto... awaiting allocated funding. |
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#3 |
Ever heard of Beech Island??
Join Date: Apr 2007
Location: Clemson, SC
Posts: 327
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Re: Backfire on deceleration.
It could be runnin a little rich, mine was backfiring on decel, and we put smaller jets in it and it stopped.
With a leaky exhaust, oxygen gets sucked in and mixes with unburnt fuel, and ignites because the headers are really hot. At least thats how it was explained to me. |
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#4 |
Registered User
Join Date: Apr 2004
Location: Clanton, AL
Posts: 1,253
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Re: Backfire on deceleration.
It is more of a "poppity pop pop" in a subdued sort of way.
I wouldn't be suprised if I had an exhaust leak. It sounds slightly different on the passenser side. It could be running rich, but the 9923 model Holley that I have was supposed to be a replacement for a 77 K-10 w/ 350 & I'm running it on a 400sbc. Maybe I'll play w/ the jets when I take it apart to adjust the floats. You gotta love stalling out when you stop too fast. ![]()
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85' CUCV M1009!!! The newest addition! 6.2diesel, Th400, NP208, & only 36k miles! 70' C-10 LWB Fleetside - Looking good these days! ![]() 05' Dodge Neon 88' Winner Escape Sport 1750 - 4.3v6 94' Seadoo SP 84' Honda TRX200, bare nekkid. Just a frame & tires. Always looking for another project or any good deal! ![]() |
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#5 |
Registered User
Join Date: Nov 2006
Location: Portland
Posts: 382
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Re: Backfire on deceleration.
Mine does the poppity pop pop on decel. I was told that I was running a little rich or that the timing may be a little off.
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#6 |
Registered User
Join Date: Apr 2004
Location: Clanton, AL
Posts: 1,253
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Re: Backfire on deceleration.
I think I already know the answer to this, but does the poppity pop pop on decel hurt anything?
__________________
85' CUCV M1009!!! The newest addition! 6.2diesel, Th400, NP208, & only 36k miles! 70' C-10 LWB Fleetside - Looking good these days! ![]() 05' Dodge Neon 88' Winner Escape Sport 1750 - 4.3v6 94' Seadoo SP 84' Honda TRX200, bare nekkid. Just a frame & tires. Always looking for another project or any good deal! ![]() |
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#7 |
Registered User
Join Date: Jan 2006
Location: Morro Bay C.A
Posts: 179
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Re: Backfire on deceleration.
nope doesnt hurt a thing. and even turbo mufflers with headers will pop... you cant get around it... a balace pipe or h pipe might help a little
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'69 C10 SWB 350 edelbrock int/carb .420 .442 cam 700r4 70 lwb c10 work truck 250 I6 with two inch lift springs and a two inch bodylift and 33/12.5s |
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#8 | |
Registered User
Join Date: Oct 2006
Location: Cheney, KS
Posts: 669
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Re: Backfire on deceleration.
What kind of exhaust are you running? Might be a back pressure problem. Had that for happening for awhile on the V8 S-10 with straight pipes.
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#9 | |
Senior Member
![]() Join Date: Dec 2006
Location: Studio City, Calif.
Posts: 2,873
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Re: Backfire on deceleration.
Cheyenne says;
Quote:
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'69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme. |
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#10 |
Senior Member
Join Date: Mar 2007
Location: VANCOUVER ISLAND, B.C., Canada
Posts: 1,602
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Re: Backfire on deceleration.
I would be inclined to check the timing. It may need to be advanced a hair. When you shut your truck off does it just stop or does it run on? If it does not run on then you can advance it a little. Also how's the timing belt? Can you turn the engine by hand? It might have jumped a cog. If the battery/tach wires to the dist. are not at the 3 oclock position and appear to be more at the 2 oclock position and you can turn engine by hand the timing chain could have jumped a cog. This would cause the timing to be out. If this is the case retime the engine.
esll. ![]() * Hope this helps at least it's free and does not take long.
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Tomorrow is for doing today's stuff. ![]() ************************************* This truck has just the right amount of energy for me! ![]() Author: Warren Lake .... (member) Last edited by EAST SIDE LOW LIFE; 05-11-2007 at 02:42 AM. |
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#11 | |
Registered User
Join Date: Apr 2004
Location: Clanton, AL
Posts: 1,253
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Re: Backfire on deceleration.
Quote:
Its very possible that I may need to advance a little timing, but as it sits, I can't advance the timing any further b/c the vacuum advance canister is running into the intake. The exhaust is cheap headers going into about 2.25" dual pipes w/ glass packs (no glass left in them).
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85' CUCV M1009!!! The newest addition! 6.2diesel, Th400, NP208, & only 36k miles! 70' C-10 LWB Fleetside - Looking good these days! ![]() 05' Dodge Neon 88' Winner Escape Sport 1750 - 4.3v6 94' Seadoo SP 84' Honda TRX200, bare nekkid. Just a frame & tires. Always looking for another project or any good deal! ![]() |
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#12 |
Registered User
Join Date: Nov 2006
Location: lafayette, ga.
Posts: 832
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Re: Backfire on deceleration.
I Myself Would Play With The Timing. Thats Where I Have Heard The Most Popping Or Backfiring In Any System.
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![]() It must be fought for, protected, and handed on for them to do the same.... Ronald Reagan |
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#13 |
Registered User
Join Date: Dec 2006
Location: New Mexico
Posts: 88
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Re: Backfire on deceleration.
I'd call that one muffler noise. Glasspacks pop and rasp and all that stuff. I think that's prolly where its coming from. Justin
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'72 C/10 SWB 355/ broken TH350 and 3.07s ![]() ![]() |
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#14 |
Super Trucker
Join Date: May 2005
Location: Newnan, Georgia
Posts: 201
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Re: Backfire on deceleration.
i've got the same basic problem. but i have a nice little exhaust leak. i was wondering if the large explosion in the tailpipe would somehow hurt my valves. i guess not. mine sounds like a shotgun going off under the truck. a friend of mine was following me one night when it happened. he said i had a flame shoot out the tailpipe. if wonder if i could be running a little rich? i'm kinda sure my timing has retarded itself again.
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American by birth, Southern by the grace of God. ".. you aint just whistlin' Dixie..." Rebel till I die |
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#15 |
Registered User
Join Date: Dec 1999
Location: Old Mission, MI, USA
Posts: 2,510
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Re: Backfire on deceleration.
Leaky exhaust causes this. It would be foolish to try anything else before you fix the exhaust leaks.
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Tim |
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#16 | |
Senior Member
![]() Join Date: Dec 2006
Location: Studio City, Calif.
Posts: 2,873
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Re: Backfire on deceleration.
Quote:
![]() 100% good advice.
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'69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme. |
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#17 |
Registered User
Join Date: Jan 2007
Location: South Florida
Posts: 158
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Re: Backfire on deceleration.
Advance the timing!
After I rebuilt my 350 it would backfire pretty bad on deceleration just like you have described. The timing was set at stock specs but it now had a mild cam. It ran great but had the backfire. After about 300 miles I got tired of hearing the backfire, although it did sound pretty cool! So I advanced the timing like 8 degrees and the backfire is 99% gone and it has much more power. Rob |
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#18 |
Registered User
Join Date: Apr 2004
Location: Clanton, AL
Posts: 1,253
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Re: Backfire on deceleration.
Although I still need to look at the timing, I found the problem.
I used the cheapo exhaust gaskets from the cheapo MrGasket engine kit. #6 is definately leaking. It got worse today, to the point that when it backfires, I can see the spark (looking right at it, of course). Don't you love when problems identify themselves, in a non-desctuctive manner!.
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85' CUCV M1009!!! The newest addition! 6.2diesel, Th400, NP208, & only 36k miles! 70' C-10 LWB Fleetside - Looking good these days! ![]() 05' Dodge Neon 88' Winner Escape Sport 1750 - 4.3v6 94' Seadoo SP 84' Honda TRX200, bare nekkid. Just a frame & tires. Always looking for another project or any good deal! ![]() |
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#19 |
It's the air up here
Join Date: Mar 2007
Location: Western Colorado
Posts: 710
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Re: Backfire on deceleration.
On deceleration, pop and bang can be caused from exhaust valve not sealing or being adjusted too tight. Have you tried running the rack? Or maybe a compression check?
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1971 cheyenne k10 4 speed np 205 454 BB factory a/c ![]() Never drink down stream of the herd! Eric Last edited by eedgell; 05-12-2007 at 05:13 PM. |
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#20 |
Registered User
Join Date: Apr 2007
Location: Kelso, WA
Posts: 363
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Re: Backfire on deceleration.
I've also got the full length headers and 2.5" glassies... deffinitly will pop on decel. Check the timing (mines at 8 degrees), also wouldn't hurt to check the plugs and wires. Mine still pops mildly but it runs like a champ!
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1972 K10 4" lift, 35’s, 383 stroker, 700r4, current project. 1971 C10 3"/5" static drop, 355/350. "La Bamba.” The current daily driver. |
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#21 |
Senior Member
Join Date: Mar 2007
Location: VANCOUVER ISLAND, B.C., Canada
Posts: 1,602
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Re: Backfire on deceleration.
I've always had good results using Percy's Life time gaskets. They cost a little more but well worth it.
esll. ![]()
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Tomorrow is for doing today's stuff. ![]() ************************************* This truck has just the right amount of energy for me! ![]() Author: Warren Lake .... (member) |
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#22 |
Registered User
Join Date: Feb 2007
Location: Central Florida
Posts: 166
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Re: Backfire on deceleration.
mine has the same sound with cheap headers and 2 1/4 dual exhaust. I know my problem is the fact I used the cheap collector gaskets that dont seal for anything. I will be using copper ones from now on. Dont waste your money on the crappy cardboard style ones.
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#23 | |
Registered User
Join Date: Apr 2004
Location: Clanton, AL
Posts: 1,253
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Re: Backfire on deceleration.
Quote:
& I also have collector gaskets, which I've never used before. I always just went without.
__________________
85' CUCV M1009!!! The newest addition! 6.2diesel, Th400, NP208, & only 36k miles! 70' C-10 LWB Fleetside - Looking good these days! ![]() 05' Dodge Neon 88' Winner Escape Sport 1750 - 4.3v6 94' Seadoo SP 84' Honda TRX200, bare nekkid. Just a frame & tires. Always looking for another project or any good deal! ![]() |
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#24 |
Registered User
Join Date: Jan 2007
Location: Goldsby,OK
Posts: 324
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Re: Backfire on deceleration.
my buddys truck did the same thing if you floored it and then let off it would pop a few times. he only had one gasket under the carb. he then put a new gasket with a plate between and a secound gasket. did doesnt pop anymore.
his dad said that sometimes the intake manifolds dont have a flat surface and it seals better like the way he did it. |
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