Backfiring - The 1947 - Present Chevrolet & GMC Truck Message Board Network

pyro883 · 07-16-2007, 08:28 PM

My truck is starting up and it hesitats when starting. Once it goes it runs great above 2500 rpms but bellow that it starts spitting and sputtering and backfires. Any ideas? Also the engine will not idle if you let off the gas the engine dies out

manimal · 07-16-2007, 08:37 PM

Check the timing. Is it backfiring through the carb or exhaust?

pyro883 · 07-16-2007, 08:38 PM

through the carb. I cant even get the truck to idle to time it.

nplocoman · 07-16-2007, 08:51 PM

Pull plug wires off one at a time. Could be an intake valve?

grayw0lf · 07-16-2007, 09:36 PM

1. I'd check timing, by hand if not by light.
2. I'd consider the age/condition of the carb. Is this an existing problem that got worse, or a new problem on a newer setup?

jhow66 · 07-16-2007, 10:31 PM

Carb flooding. Do you see black smoke?

Blazer1970 · 07-17-2007, 06:39 AM

Plug wires crossed?

Pumped 68 · 07-17-2007, 07:23 AM

I would check timing, Also inspect the cap and rotor for cracks or excessive wear. Check and make sure your idle is right, You can also Try retarding your timing by hand & Ear.

You can also pull your plugs to help let you know if you're running a little too rich or lean (Grey / Ash color Lean, Black or sometimes gasoline smell is Rich, Tan is just right), and make sure your Firing order is right (more then likely it is but just in case.) Float Level Too High?

That should help, Need more info on the truck to get a better idea on whats going on. Sounds like the timing is too far advanced though and the idle might be off a bit, More info would help though.

manimal · 07-17-2007, 08:38 AM

Sound like one of a few things. The timing is too far advanced,the chain is streched, cam is going flat, or the wires are in the wrong places. Firing order is 1-8-4-3-6-5-7-2 clockwise. Good Luck. Keep us posted.
-Lance

OLDCHEV4X4 · 07-17-2007, 08:51 AM

Backfireing through the carb is a lean condition or timing is late. Advance the timing not retard it.
Make sure all ignition parts are good and carb accelerator pump is working.
I would start there first.

*Billla* · 07-17-2007, 09:40 AM

Agreed that job 1 is timing. If you can't get the truck to idle, you can statically set the timing by positioning the engine at 8-12 degrees BTDC and pointing the rotor exactly at the #1 tower.

If you can't get it to idle, I'd also take a hard look for vacuum leaks.

pyro883 · 07-17-2007, 07:23 PM

Well I advanced the distributor to get the best time i could get with just sound. Now its no longer back firing. I also think I found a vaccum leak there are 2 black gaskets on the carburator, and I sprayed starter fluid and it reved up when i sprayed it along the gaskets. So I went to autozone and they dont have any gaskets for it saying it was part of a rebuild kit. Dont know if there is any truth to this. Do any of you guys know and if so where do you get them at. I belive I have quadrajet carb. Thanks for any help.

*Billla* · 07-17-2007, 08:02 PM

Just a reminder that your timing is almost certainly too far advanced at this point.

They're likely right about the gaskets; the rebuild kit should be fairly cheap...

tx71smallblock · 07-17-2007, 08:12 PM

after all that i have seen it sounds like your timing is a full 180 out and a new set of plugs is recomended. you should be able to tell if u have a quadrajet, its an almost perfect square and looks like no other carb out there.

mnunn454 · 07-17-2007, 08:55 PM

Sounds like a vacuum leak to me.

*Billla* · 07-17-2007, 09:02 PM

Quote:

Originally Posted by tx71smallblock

after all that i have seen it sounds like your timing is a full 180 out and a new set of plugs is recomended. you should be able to tell if u have a quadrajet, its an almost perfect square and looks like no other carb out there.

I can't see how it's 180 out if it's running smooth above 2500 RPM...

pyro883 · 07-17-2007, 09:03 PM

Well here is pictures of the carb that I have. Maybe someone can help with me with what kind it is. I am not sure I am new and dumb to all of this.

k2c1959 · 07-17-2007, 09:23 PM

Quote:

Originally Posted by pyro883

Well here is pictures of the carb that I have. Maybe someone can help with me with what kind it is. I am not sure I am new and dumb to all of this.

Attachment 283435

that be a good ole quadrajet ye got there

pyro883 · 07-17-2007, 09:36 PM

anybody know where i could get a rebuild for it?

**WorkinLonghorn** · 07-17-2007, 10:26 PM

Looking at that picture, the first thing I notice is that the clamp is missing from the PCV hose where it fits on the carb.That is the larger black hose that goes into the front center of the carb and from there to the PVC valve in the left valve cover. Chech those connections and add a clamp. If the hose is hardened where it fits,replace it! Pull the PVC valve out and shake it to see if it "rattles" (it should,if not REPLACE IT!). Spraying starer fluid around the carb will always cause an increase in RPM because it will be sucked into the carb,it's kind of a tricky test. If you had the air cleaner on and the gaskets under that were good,then the test would be more meaningful as the spray would be less likely to find it's way into the air-cleaner intake.

manimal · 07-18-2007, 12:08 AM

Quote:

Originally Posted by manimal

Sound like one of a few things. The timing is too far advanced,the chain is streched, cam is going flat, or the wires are in the wrong places. Firing order is 1-8-4-3-6-5-7-2 clockwise. Good Luck. Keep us posted.
-Lance

Wow, what was I thinking!? I was meaning retarded and put it adavanced! Now I am retarded! Sorry!

cell · 07-18-2007, 12:09 AM

I had a backfiring problem like you, and it was because of a lean condition (vacuum leak).

68speedalert · 07-18-2007, 12:40 PM

By the look of the carb in the pic, it looks like it's a recent rebuild. Check the accelerator pump for proper operation. Engine off, look down the front 2 barrels while slowly opening throttle all the way open. You should see a continuous stream of gas shooting into each front barrel. It's real common for rebuilds to have that circuit not working right.

That won't affect idle, however. Try holding your hand over the front barrels and see it it helps it to idle. It it helps, possible idle mixture problem or could have a problem with the idle circuits in the carb.

prg machine · 07-18-2007, 02:02 PM

vaccuuum leaak

jhow66 · 07-18-2007, 02:30 PM

Quote:

Originally Posted by pyro883

Well here is pictures of the carb that I have. Maybe someone can help with me with what kind it is. I am not sure I am new and dumb to all of this.

Attachment 283435

You have a complete vacumn leak on this pipe. close it off.

07-16-2007, 08:37 PM	#2
manimal Registered User Join Date: Oct 2006 Location: Southern Kalifornia Posts: 3,066	Re: Backfiring Check the timing. Is it backfiring through the carb or exhaust? __________________ ------------------------------------------------------ -Lance 2000 Chevy C2500, 5.7, 4L80e 1999 Suburban K2500, 7.4, 4L80e 1980 Camaro..son's car...PROJECT

07-16-2007, 08:38 PM	#3
pyro883 Registered User Join Date: Jan 2007 Location: Beaufort, SC Posts: 244	Re: Backfiring through the carb. I cant even get the truck to idle to time it.

07-16-2007, 08:51 PM	#4
nplocoman never finish start another Join Date: Mar 2007 Location: North Platte Nebraska Posts: 432	Re: Backfiring Pull plug wires off one at a time. Could be an intake valve?

07-16-2007, 09:36 PM	#5
grayw0lf Registered User Join Date: Apr 2004 Location: Clanton, AL Posts: 1,253	Re: Backfiring 1. I'd check timing, by hand if not by light. 2. I'd consider the age/condition of the carb. Is this an existing problem that got worse, or a new problem on a newer setup? __________________ 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!

07-16-2007, 10:31 PM	#6
jhow66 Registered User Join Date: Jun 2002 Location: Knoxville Tenn. Posts: 3,058	Re: Backfiring Carb flooding. Do you see black smoke? __________________ 56 Chevy Bel-Air 2dr. HT (purchased new) 71 Chevy Cheyenne SWB PU (502HO) 65 GMC short bed step--work in progress and my gofer

07-16-2007, 08:28 PM	#1
pyro883 Registered User Join Date: Jan 2007 Location: Beaufort, SC Posts: 244	Backfiring My truck is starting up and it hesitats when starting. Once it goes it runs great above 2500 rpms but bellow that it starts spitting and sputtering and backfires. Any ideas? Also the engine will not idle if you let off the gas the engine dies out Last edited by pyro883; 07-16-2007 at 08:33 PM.

07-17-2007, 06:39 AM	#7
Blazer1970 Registered User Join Date: Dec 1999 Location: Old Mission, MI, USA Posts: 2,510	Re: Backfiring Plug wires crossed? __________________ Tim

07-17-2007, 07:23 AM	#8
Pumped 68 Registered User Join Date: Jul 2007 Location: Lancaster, CA Posts: 6	Re: Backfiring I would check timing, Also inspect the cap and rotor for cracks or excessive wear. Check and make sure your idle is right, You can also Try retarding your timing by hand & Ear. You can also pull your plugs to help let you know if you're running a little too rich or lean (Grey / Ash color Lean, Black or sometimes gasoline smell is Rich, Tan is just right), and make sure your Firing order is right (more then likely it is but just in case.) Float Level Too High? That should help, Need more info on the truck to get a better idea on whats going on. Sounds like the timing is too far advanced though and the idle might be off a bit, More info would help though. Last edited by Pumped 68; 07-17-2007 at 07:25 AM.

07-17-2007, 08:38 AM	#9
manimal Registered User Join Date: Oct 2006 Location: Southern Kalifornia Posts: 3,066	Re: Backfiring Sound like one of a few things. The timing is too far advanced,the chain is streched, cam is going flat, or the wires are in the wrong places. Firing order is 1-8-4-3-6-5-7-2 clockwise. Good Luck. Keep us posted. -Lance __________________ ------------------------------------------------------ -Lance 2000 Chevy C2500, 5.7, 4L80e 1999 Suburban K2500, 7.4, 4L80e 1980 Camaro..son's car...PROJECT

07-17-2007, 08:51 AM	#10
OLDCHEV4X4 Registered User Join Date: Apr 2000 Location: Chester County PA, USA Posts: 1,228	Re: Backfiring Backfireing through the carb is a lean condition or timing is late. Advance the timing not retard it. Make sure all ignition parts are good and carb accelerator pump is working. I would start there first. __________________ 72 k/10 short wideside 72 k/5 Blazer 72 K/5 Blazer - obstacle course racer 72 custom/10 8' bed 70 K/5 Blazer 67 C-30 GMC Flat bed 86 GMC K2500 Plow Truck 85 GMC K3500 84 M1008 85 M1008 77 G30 Motor Home 72 Sears 12hp Tractor Glenmoore, Pennsylvania

07-17-2007, 09:40 AM	#11
Billla Account Suspended Join Date: Sep 2005 Location: Monroe, WA Posts: 3,815	Re: Backfiring Agreed that job 1 is timing. If you can't get the truck to idle, you can statically set the timing by positioning the engine at 8-12 degrees BTDC and pointing the rotor exactly at the #1 tower. If you can't get it to idle, I'd also take a hard look for vacuum leaks.

07-17-2007, 07:23 PM	#12
pyro883 Registered User Join Date: Jan 2007 Location: Beaufort, SC Posts: 244	Re: Backfiring Well I advanced the distributor to get the best time i could get with just sound. Now its no longer back firing. I also think I found a vaccum leak there are 2 black gaskets on the carburator, and I sprayed starter fluid and it reved up when i sprayed it along the gaskets. So I went to autozone and they dont have any gaskets for it saying it was part of a rebuild kit. Dont know if there is any truth to this. Do any of you guys know and if so where do you get them at. I belive I have quadrajet carb. Thanks for any help.

07-17-2007, 08:02 PM	#13
Billla Account Suspended Join Date: Sep 2005 Location: Monroe, WA Posts: 3,815	Re: Backfiring Just a reminder that your timing is almost certainly too far advanced at this point. They're likely right about the gaskets; the rebuild kit should be fairly cheap...

07-17-2007, 08:12 PM	#14
tx71smallblock Registered User Join Date: Jun 2007 Location: Georgetown , TX Posts: 166	Re: Backfiring after all that i have seen it sounds like your timing is a full 180 out and a new set of plugs is recomended. you should be able to tell if u have a quadrajet, its an almost perfect square and looks like no other carb out there. __________________ 70 Chevy C-10 short step 250/3 on the tree Owned: 71 longbed Cheyenne 70 swb

07-17-2007, 08:55 PM	#15
mnunn454 Registered User Join Date: Dec 2006 Location: Rhode Island Posts: 602	Re: Backfiring Sounds like a vacuum leak to me. __________________ 69 C-10 SWB Fleetside 454/TH400 (For Sale) http://67-72chevytrucks.com/vboard/c...ct=1299&cat=12 70 El Camino 454 TH700 29 Model A 4-DR Sedan Deluxe



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07-17-2007, 09:03 PM	#17
pyro883 Registered User Join Date: Jan 2007 Location: Beaufort, SC Posts: 244	Re: Backfiring Well here is pictures of the carb that I have. Maybe someone can help with me with what kind it is. I am not sure I am new and dumb to all of this.

07-17-2007, 09:36 PM	#19
pyro883 Registered User Join Date: Jan 2007 Location: Beaufort, SC Posts: 244	Re: Backfiring anybody know where i could get a rebuild for it?

07-17-2007, 10:26 PM	#20
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,861	Re: Backfiring Looking at that picture, the first thing I notice is that the clamp is missing from the PCV hose where it fits on the carb.That is the larger black hose that goes into the front center of the carb and from there to the PVC valve in the left valve cover. Chech those connections and add a clamp. If the hose is hardened where it fits,replace it! Pull the PVC valve out and shake it to see if it "rattles" (it should,if not REPLACE IT!). Spraying starer fluid around the carb will always cause an increase in RPM because it will be sucked into the carb,it's kind of a tricky test. If you had the air cleaner on and the gaskets under that were good,then the test would be more meaningful as the spray would be less likely to find it's way into the air-cleaner intake. __________________ '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, 2nd owner, Work-Truck supreme. Best $300 I ever spent. Last edited by WorkinLonghorn; 07-17-2007 at 10:26 PM.

07-18-2007, 12:09 AM	#22
cell Active Member Join Date: Nov 2005 Location: Austin Posts: 217	Re: Backfiring I had a backfiring problem like you, and it was because of a lean condition (vacuum leak).

07-18-2007, 12:40 PM	#23
68speedalert Semi-Senior Member Join Date: Feb 2003 Location: Washington, MO Posts: 390	Re: Backfiring By the look of the carb in the pic, it looks like it's a recent rebuild. Check the accelerator pump for proper operation. Engine off, look down the front 2 barrels while slowly opening throttle all the way open. You should see a continuous stream of gas shooting into each front barrel. It's real common for rebuilds to have that circuit not working right. That won't affect idle, however. Try holding your hand over the front barrels and see it it helps it to idle. It it helps, possible idle mixture problem or could have a problem with the idle circuits in the carb. __________________ '68 50th Anniversery LWB Custom Fleetside, '77 305 v-8, turbo 350, factory speed alert (still works), '71 drivetrain w/front discs. Some call it ugly yellow, others call it Ochre '83 Jeep CJ-7, <80,000 miles, original owner '04 Chevy Suburban, 4x4, 2500, 6.0 ltr, Predator Diablo programmer '95 GMC K1500 Extended cab

07-18-2007, 02:02 PM	#24
prg machine Registered User Join Date: Jun 2007 Location: oregon Posts: 1,069	Re: Backfiring vaccuuum leaak

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