1980 Corvette Backfire through carb.
Thread Starter
Intermediate
Joined: Oct 2008
Posts: 33
Likes: 0
From: Norfolk Virginia
1980 Corvette Backfire through carb.
Hi everyone,
I have a 1980 vette she starts and runs great for 10 to 15 minutes. After that point she starts backfiring/stalls and becomes hard to start again. So far i changed plugs, wires, rotor and cap. I ended up putting a new fuel line in from the pump to the carb because i changed the filter but twisted the line when loosening the nut. Fuel flow seems normal at least the same as my camaro. I also swapped out the q-junk with the one from my camaro and had the same problem. I don't hear and vacuum leaks on the vette. The way she is acting is that once the choke is fully open there is too much air getting in but i can't figure this out. Any insight would be greatly appreciated.
Hamilton
I have a 1980 vette she starts and runs great for 10 to 15 minutes. After that point she starts backfiring/stalls and becomes hard to start again. So far i changed plugs, wires, rotor and cap. I ended up putting a new fuel line in from the pump to the carb because i changed the filter but twisted the line when loosening the nut. Fuel flow seems normal at least the same as my camaro. I also swapped out the q-junk with the one from my camaro and had the same problem. I don't hear and vacuum leaks on the vette. The way she is acting is that once the choke is fully open there is too much air getting in but i can't figure this out. Any insight would be greatly appreciated.
Hamilton
Instructor
Joined: Jan 2006
Posts: 228
Likes: 1
From: Warwick New York
I would tend to agree that it is a exhaust lobe but since it doesn't happen when it cold it could be a bad hydraulic lifter that leaks out when it warms up but is still has to be on the exhaust side. That the way you get a backfire. If it was on the intake side you would only loose the power because when the intake doesn't open no gas gets into the cylinder.
Drifting
Joined: Apr 2006
Posts: 1,911
Likes: 2
From: gold coast queensland
have you done any head work such as had them machined . your pushrod geometry will now be different and some if not all valves may not be closing properly creating a backfire through carb.
Pro
Joined: Jan 2009
Posts: 576
Likes: 3
Sounds like my problem;
http://forums.corvetteforum.com/c3-t...-cam-lobe.html
Pretty much convinced it's the cam now.
http://forums.corvetteforum.com/c3-t...-cam-lobe.html
Pretty much convinced it's the cam now.
Abby Normal
Joined: Dec 2001
Posts: 53,394
Likes: 92
From: Press "2" for English
sounds like a good starting point to me. just a heads up to others that own late 70's, early 80's SBC, the cams are pretty much junk. 50k to 75k miles is about as good as you're going to get before they start wiping lobes. i think it may actually be more a result of the changes in oil composition (less zinc, we've been down this road before with flat tappet cams) than the actual cam.
i wiped my original cam around 75k miles. by the time i finally pulled it out for an overhaul at 110k miles, i had 5 lobes in various stages of 'gone'
i wiped my original cam around 75k miles. by the time i finally pulled it out for an overhaul at 110k miles, i had 5 lobes in various stages of 'gone'
Racer
Joined: Mar 2008
Posts: 428
Likes: 0
Backfiring through the carb after the engine runs awhile? I had this before and I finally found a broken intake valve spring. It was broken near the top and very hard to see. It didn't backfire consistently, the valve obviously worked some of the time just well enough and other times it backfired.
Last edited by Project80; Feb 17, 2009 at 11:55 PM.
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Instructor
Joined: Sep 2008
Posts: 222
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From: penngrove ca
Generally, backfire thru carb is intake problem. I would check carb & intake 1st since it's easist and costs the least to check. Then I'd pull valve covers and check rocker arms. I had similar problem on 76, turned out to be loose nut on rocker arm.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
Backfire through the carb is [usually] a timing problem. If the carb didn't backfire when it was cold, it wouldn't be a carb problem. And I've never experienced a backfire caused by an intake manifold. 
Drifting
Joined: Jul 1999
Posts: 1,542
Likes: 43
From: Plano TX
I've had this happen to a couple of cars running the GM HEI ignition. So before going after a mechanical failure try replacing the module as previously suggested. In fail mode they seem to be fine until the heat gets into the distributor housing. A mechanical failure usually will show up right away not after it's warmed up.
The HEI rotor is notorius for burning through the center if not changed often enough. You'll see orange like deposits under the rotor and on the mechanical advance mechanism cause by arcing. These stray arcs can hit the module causing it to fail over time.
Hope it's a simple fix
The HEI rotor is notorius for burning through the center if not changed often enough. You'll see orange like deposits under the rotor and on the mechanical advance mechanism cause by arcing. These stray arcs can hit the module causing it to fail over time.
Hope it's a simple fix
Instructor
Joined: Jun 2005
Posts: 135
Likes: 0
I've read all of the posts - my initial reaction is too lean a mixture in the carburetor. When the engine is cold, the choke enriches the mixture - once warm, the choke is wide open and a leaner mixture is provided. I would turn the two idle mixture screws OUT a complete turn and see if this mitigates the problem. It may be a simple air/fuel mixture problem.
Instructor
Joined: Jan 2009
Posts: 117
Likes: 0
Sorry if this a little bit of a hijack of the post, but it is related, just slightly different scenario. I too get the occasional blow back through the carb.
I rebuilt the top end and re-lapped all of the valves (intake and exhaust) and then had it set up on a dyno (I never removed the cam). I hadnt driven it for a few months and also fitted a new gearbox and a rear diff at the same time.
Since all of this was done if I give it full throttle at lower revs (below 2,500) probably one in every 15 times it will blow back through the carb. I know the mixture is right so I put it down to excess load and not enough low down power to pull cleanly. Will the revs being too low and excessive load cause this or has it got to be an ignition/timing or intake/fuelling problem?
Thanks in advance,
Lester
I have a new distributor and igntion system ready to go in, but it wont be back on the road for a few months.
I rebuilt the top end and re-lapped all of the valves (intake and exhaust) and then had it set up on a dyno (I never removed the cam). I hadnt driven it for a few months and also fitted a new gearbox and a rear diff at the same time.
Since all of this was done if I give it full throttle at lower revs (below 2,500) probably one in every 15 times it will blow back through the carb. I know the mixture is right so I put it down to excess load and not enough low down power to pull cleanly. Will the revs being too low and excessive load cause this or has it got to be an ignition/timing or intake/fuelling problem?
Thanks in advance,
Lester
I have a new distributor and igntion system ready to go in, but it wont be back on the road for a few months.
Drifting
Joined: Apr 2006
Posts: 1,911
Likes: 2
From: gold coast queensland
i have. when heads got machined on a friends car it made the V that the manifold sits in a different size as the heads now moved in further. the manifold gaskets then did not seal as they should between the head and manifold. caused backfire. he took off manifold and used rtv around ports and there was no more backfire.
Last edited by gingerbreadman1977; Feb 19, 2009 at 12:31 AM.
Drifting
Joined: Apr 2006
Posts: 1,911
Likes: 2
From: gold coast queensland
I've read all of the posts - my initial reaction is too lean a mixture in the carburetor. When the engine is cold, the choke enriches the mixture - once warm, the choke is wide open and a leaner mixture is provided. I would turn the two idle mixture screws OUT a complete turn and see if this mitigates the problem. It may be a simple air/fuel mixture problem.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
A defective intake gasket could certainly cause backfiring. So, why is it not backfiring when cold? Does the intake gasket shrivel up from the cold and somehow seal itself up??? Symptoms still tell me the module in the dizzy is bad.
Instructor
Joined: Jan 2006
Posts: 228
Likes: 1
From: Warwick New York
I think you should determine if the backfire is in a specific cylinder and is consistent at an idle or just above an idle. If you find a signal cylinder it is something related to that cylinder. I could be alot of things but if your motor has a good amount of miles on it there is a good chance it could be valve related. Ignition back fire I have found are usually more difficult to nail down to a specific cylinder. This also applys to carb and vacuum problems.
Le Mans Master
Joined: Jun 2008
Posts: 5,284
Likes: 235
From: Rockledge FL
i read that an intake manifold leak will cause a very very low vacuum reading on a gauge.
what are you getting for idle vacuum
also do you have one of those diverter valves in your exhaust that might be getting stuck closed?
what are you getting for idle vacuum
also do you have one of those diverter valves in your exhaust that might be getting stuck closed?