Help! Carburetor is backfiring! - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**pjmoreland** · 04-10-2005, 02:03 AM

When I rev my engine up slowly, the carburetor starts backfiring repeatedly. It makes a pop-pop-pop-pop noise in rhythm with the RPM of the engine.

It just started doing this today when I was driving down the highway. At first it only happened with the throttle barely open. It got progressively worse over the course of about 10 miles to the point where I thought I wasn't going to make it home.

I thought that maybe a valve had gone bad so I disconnected one spark plug wire at a time to see if there was one cylinder in particular that was causing the problem, but in no cases did the popping go away.

I checked my timing and it is set at 10 degrees. Just as a test I tried retarding the timing to about 2 degrees, but the popping persisted.

The manifold vacuum is around 15 at idle.

I have a mild 350 engine with a four-barrel Quadrajet and an HEI distributor. I triple-checked my spark plug wires. They are definitely arranged in the 18436572 order.

Any ideas?

Alexis · 04-10-2005, 02:11 AM

check if you have any spark plugs loose.

**pjmoreland** · 04-10-2005, 02:37 AM

I went outside just now and checked the spark plugs. They were all tight.

pritch · 04-10-2005, 03:10 AM

Maybe a piece of carbon or some other junk causing a valve not to close all the way? Maybe check compression.

JimKshortstep4x4 · 04-10-2005, 06:40 AM

The way the problem progressed I would suggest pulling the valve covers to make sure the valves are going up and down and that the springs are all good.

Jim

Brainchild · 04-10-2005, 08:23 PM

Quote:

Originally Posted by JimKshortstep4x4

The way the problem progressed I would suggest pulling the valve covers to make sure the valves are going up and down and that the springs are all good.

Jim

I agree,sounds like a bent pushrod,broken rocker arm,valve or even a lobe off the cam to me.Hopefully it's not real serious.

**breeh** · 04-10-2005, 09:15 PM

I just had the same exact thing happen to my '68. Pulled the valve covers and found that I had lost a cam lobe.

tmcgrawlookalike · 04-10-2005, 08:01 AM

Sounds Like A Stuck/burnt Valve

Mudder · 04-10-2005, 09:47 AM

Compression test time.

Gordo · 04-10-2005, 10:07 AM

Also check inside the distributor cap for carbon tracks, evidence of spark jumping between contacts. I also have seen the wrong screws (to long) installed to hold in the coil and they push/break thru into the cap creating problems.

**pjmoreland** · 04-10-2005, 12:34 PM

I will do a compression test and will check out the distributor tomorrow. Thank you for the suggestions.

willow72 · 04-10-2005, 04:32 PM

maybe the timing chain going out

**pjmoreland** · 04-11-2005, 11:55 PM

Well I just got done doing a compression test. six of the cylinders measured at around 135 psi. Cylinder 6 measured at 45 psi, and cylinder 8 measured at 70 psi. I popped my valve cover off and watched the rockers as I cranked the engine over. They were moving up and down just fine, and I could spin the pushrods during the compression stroke, so the valves are closing all the way. I'm guessing I have a couple burned valves. Time to pull the heads off. What a bummer!

68cajun · 04-12-2005, 12:20 AM

Are you sure the rockers are moving all the same distance. If you loss a cam lobe it will move but not as much as the others.

**VetteVet** · 04-12-2005, 12:33 AM

I vote for a blown head gasket between 6 and 8. Seems like burnt valves would have a little more compression. Good luck.
VetteVet

**pjmoreland** · 04-12-2005, 03:20 AM

I just got done pulling the head off. The intake valves on cylinders 6 and 8 are split. The head gasket looks like it is in good shape. Any suggestions on what I should do to fix this? Am I asking for trouble if I only replace one head?

OLDCHEV4X4 · 04-12-2005, 06:52 AM

those are exaust valves

JimKshortstep4x4 · 04-12-2005, 09:42 AM

I think that I would have the valves reground/replaced on both heads. You could just replace the two exhaust valves, (regrinding the seats).

One thing that bothers me is that you should not get backfiring through the carburetor with a couple of burned exhaust valves. I still suspect a broken spring or a bad cam.

Did you check the intake valves on the two cylinders?

Jim

**VetteVet** · 04-12-2005, 11:04 AM

It looks like the area between the exhaust valves where the head gasket was is burned away from the head gasket being blown to start with, which let's me believe that the gasket was blown to start with and it just got worse. The popping through the carb might have been heard when no. 6 intake was open as no. 8 fired and vice versa. I can't remember the valve timing so that may not be the case. I've got an old cam that I will look at later out of curiosity. Iwould make sure the head wasn't warped also. It might be quicker and easier to just slap another good head on and go. Just my thoughts

**pjmoreland** · 04-12-2005, 11:11 AM

Is there a quick way to check the cam without removing it?

**pjmoreland** · 04-12-2005, 11:19 AM

So do you think it is ok just to replace one head? I just worry about the compression being uneven between the two heads.

**pjmoreland** · 04-12-2005, 11:21 AM

VetteVet, I'll bet you're right about the blown head gasket.

JimKshortstep4x4 · 04-12-2005, 12:50 PM

The area of the head in the picture between the two cylinders does not appear to be an indication of a blown head gasket. Besides that, the area shown is between cylinders 4 and 6 and no 4 cylinder had 135 psi compression according to the compression test.

I am still concerned about the popping through the carburetor, though.

Jim

**VetteVet** · 04-12-2005, 02:09 PM

Quote:

Originally Posted by JimKshortstep4x4

The area of the head in the picture between the two cylinders does not appear to be an indication of a blown head gasket. Besides that, the area shown is between cylinders 4 and 6 and no 4 cylinder had 135 psi compression according to the compression test.

I am still concerned about the popping through the carburetor, though.

Jim

Jim, now that I looked at the picture again I see you are right because the center cylinders are the two with the exhaust valves adjacent. Holding the exhaust valves open would explain the burnt valves so maybe the springs are broken or weak like you said. In any case the head needs to be checked out or replaced and I don't see a problem with replacing just the right one if he gets a good one. He can always repair that one too, but all the valves and springs on it need to be checked. I don't think a wiped lobe on the cam would cause a burnt valve and how can an exhaust valve pop back through the carb.
Pj do all the lifters rise and fall as you turn the engine over. If they do and about the same amount then I'ds say the cam is good. You can take this as far as you want but if I had it I would either replace the right head with a good one or rebuild the right one and put the engine back together.
The problem with making a cyber diagnosis is, without seeing the engine and knowing what it sounded like, It is hard to say exactly how you should proceed. With the compression you listed on the other six cylinders and the damage on the right head you probably have good rings on six and eight so repairing or replacing the right head should put you back in business. You just wnat to be sure to get the same type head I.e. 72cc chamber sam size valves etc.
VetteVet---Keep us informed

midyearvet@aol. · 04-12-2005, 12:31 PM

Probably a flat exhaust lobe on cam. Exhaust is poping back up through carb.

04-10-2005, 02:03 AM	#1
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,426	Help! Carburetor is backfiring! When I rev my engine up slowly, the carburetor starts backfiring repeatedly. It makes a pop-pop-pop-pop noise in rhythm with the RPM of the engine. It just started doing this today when I was driving down the highway. At first it only happened with the throttle barely open. It got progressively worse over the course of about 10 miles to the point where I thought I wasn't going to make it home. I thought that maybe a valve had gone bad so I disconnected one spark plug wire at a time to see if there was one cylinder in particular that was causing the problem, but in no cases did the popping go away. I checked my timing and it is set at 10 degrees. Just as a test I tried retarding the timing to about 2 degrees, but the popping persisted. The manifold vacuum is around 15 at idle. I have a mild 350 engine with a four-barrel Quadrajet and an HEI distributor. I triple-checked my spark plug wires. They are definitely arranged in the 18436572 order. Any ideas?

04-10-2005, 03:10 AM	#4
pritch Registered User Join Date: Mar 2004 Location: Hoytsville, Utah Posts: 3,365	Maybe a piece of carbon or some other junk causing a valve not to close all the way? Maybe check compression. __________________ '68 Panel Project Boogie Chillin' '68 C-10 Ol' Green

04-10-2005, 09:15 PM	#7
breeh Recovering 67-72 Addict Join Date: Oct 2000 Location: North Bend, WA Posts: 1,788	I just had the same exact thing happen to my '68. Pulled the valve covers and found that I had lost a cam lobe. __________________ 68 Short Fleet 4x4; 427; BTO Level 3 700R4; NP205; Dana 60 front and rear; 4" lift with 35's; Disc brakes; AGR variable ratio steering; factory bucket seats; factory tach and tilt. "Friends call me cruzer cuz I like fast cars and fast women"

04-10-2005, 10:07 AM	#10
Gordo Registered User Join Date: Jan 2001 Location: Killingworth, CT. USA Posts: 3,378	Also check inside the distributor cap for carbon tracks, evidence of spark jumping between contacts. I also have seen the wrong screws (to long) installed to hold in the coil and they push/break thru into the cap creating problems. __________________ 1971 C10 swb stepside 350/700R4/3.73posi (retired as of 4/22/03) 1998 S10 short bed 2002 S10 Blazer 1942 Oldsmobile 1958 Massey Harris Pony 1951 Wife Killingworth, Connecticut May those who love us, love us, any of those who do not love us, may God turn their hearts. And if God is unable to turn their hearts, may he turn their ankles so we may know them by their limping. A man who works with his hands is a laborer; a man who works with his hands and his brain is a craftsman; but a man who works with his hands and his brain and his heart is an artist.

04-12-2005, 12:20 AM	#14
68cajun 68 C/10 Join Date: Oct 2003 Location: Houma Louisiana Posts: 367	Are you sure the rockers are moving all the same distance. If you loss a cam lobe it will move but not as much as the others. __________________ (2009) GMC Sierra Denali 6.2 AWD (67 C/10 fleetside SWB) (2018GMC Yukon Denali) Wifes Car

04-10-2005, 02:11 AM	#2
Alexis Member since 2000 Join Date: Dec 2000 Location: Mountain View Ca / Mexico Posts: 7,879	check if you have any spark plugs loose.

04-10-2005, 02:37 AM	#3
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,426	I went outside just now and checked the spark plugs. They were all tight.

04-10-2005, 06:40 AM	#5
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	The way the problem progressed I would suggest pulling the valve covers to make sure the valves are going up and down and that the springs are all good. Jim

04-10-2005, 08:01 AM	#8
tmcgrawlookalike Registered User Join Date: Jun 2004 Location: warr acres,oklahoma Posts: 650	Sounds Like A Stuck/burnt Valve

04-10-2005, 09:47 AM	#9
Mudder Registered User Join Date: Feb 2001 Location: East Central, MO Posts: 11,336	Compression test time.

04-10-2005, 12:34 PM	#11
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,426	I will do a compression test and will check out the distributor tomorrow. Thank you for the suggestions.

04-10-2005, 04:32 PM	#12
willow72 Registered User Join Date: Nov 2003 Location: dallas/ftwoth Posts: 112	maybe the timing chain going out

04-11-2005, 11:55 PM	#13
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,426	Well I just got done doing a compression test. six of the cylinders measured at around 135 psi. Cylinder 6 measured at 45 psi, and cylinder 8 measured at 70 psi. I popped my valve cover off and watched the rockers as I cranked the engine over. They were moving up and down just fine, and I could spin the pushrods during the compression stroke, so the valves are closing all the way. I'm guessing I have a couple burned valves. Time to pull the heads off. What a bummer!

04-12-2005, 12:33 AM	#15
VetteVet Msgt USAF Ret Join Date: Jan 2005 Location: Kalamazoo, Michigan Posts: 8,709	I vote for a blown head gasket between 6 and 8. Seems like burnt valves would have a little more compression. Good luck. VetteVet __________________ VetteVet metallic green 67 stepside 74 corvette convertible 1965 Harley sportster 1995 Harley wide glide Growing old is hell, but it beats the alternative.

04-12-2005, 03:20 AM	#16
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,426	Found The Problem I just got done pulling the head off. The intake valves on cylinders 6 and 8 are split. The head gasket looks like it is in good shape. Any suggestions on what I should do to fix this? Am I asking for trouble if I only replace one head? Attached Images



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04-12-2005, 06:52 AM	#17
OLDCHEV4X4 Registered User Join Date: Apr 2000 Location: Chester County PA, USA Posts: 1,228	those are exaust valves __________________ 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

04-12-2005, 09:42 AM	#18
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	I think that I would have the valves reground/replaced on both heads. You could just replace the two exhaust valves, (regrinding the seats). One thing that bothers me is that you should not get backfiring through the carburetor with a couple of burned exhaust valves. I still suspect a broken spring or a bad cam. Did you check the intake valves on the two cylinders? Jim Last edited by JimKshortstep4x4; 04-12-2005 at 09:46 AM.

04-12-2005, 11:04 AM	#19
VetteVet Msgt USAF Ret Join Date: Jan 2005 Location: Kalamazoo, Michigan Posts: 8,709	It looks like the area between the exhaust valves where the head gasket was is burned away from the head gasket being blown to start with, which let's me believe that the gasket was blown to start with and it just got worse. The popping through the carb might have been heard when no. 6 intake was open as no. 8 fired and vice versa. I can't remember the valve timing so that may not be the case. I've got an old cam that I will look at later out of curiosity. Iwould make sure the head wasn't warped also. It might be quicker and easier to just slap another good head on and go. Just my thoughts __________________ VetteVet metallic green 67 stepside 74 corvette convertible 1965 Harley sportster 1995 Harley wide glide Growing old is hell, but it beats the alternative.

04-12-2005, 11:11 AM	#20
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,426	Is there a quick way to check the cam without removing it?

04-12-2005, 11:19 AM	#21
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,426	So do you think it is ok just to replace one head? I just worry about the compression being uneven between the two heads.

04-12-2005, 11:21 AM	#22
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,426	VetteVet, I'll bet you're right about the blown head gasket.

04-12-2005, 12:50 PM	#23
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	The area of the head in the picture between the two cylinders does not appear to be an indication of a blown head gasket. Besides that, the area shown is between cylinders 4 and 6 and no 4 cylinder had 135 psi compression according to the compression test. I am still concerned about the popping through the carburetor, though. Jim

04-12-2005, 12:31 PM	#25
midyearvet@aol. Registered User Join Date: Jun 2003 Location: Napa, Ca. Posts: 103	Probably a flat exhaust lobe on cam. Exhaust is poping back up through carb.

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