Backfiring through carb
Thread Starter
3rd Gear
Joined: Aug 2012
Posts: 3
Likes: 0
Backfiring through carb
I have a 1977 vette that I inherited from my uncle. It had been sitting for several years, but I have been working on it, and I have gotten it up and running. It cranks right up and idles fine, but once I press down on the gas it backfires through the carb. Once I put it in gear and go down the road, it will drive fine until I try to accelerate, and then it will backfire through the carb again. It doesn't matter if the engine is warm or cold.
I think that the problem started with my uncle when he did a carb and intake swap on the car. He installed an Edelbrock carb and intake. Is this a timing/fuel issue?? I would appreciate any help. This is driving me crazy!
I think that the problem started with my uncle when he did a carb and intake swap on the car. He installed an Edelbrock carb and intake. Is this a timing/fuel issue?? I would appreciate any help. This is driving me crazy!
Tech Contributor
Joined: Aug 1999
Posts: 14,377
Likes: 6,376
From: At my Bar drinking and wrenching in Lafayette Colorado
Before doing any carb work, make sure you check the timing and ignition system. As I state in my tuning seminars, "90% of all carb problems are ignition problems." Get a timing light and set it up per my "How to Set Timing" paper with 36 degrees total advance and initial timing in the 8 - 18 degree range to produce the 36 total. Make sure vacuum advance is hooked up and operational, and make sure there are no vacuum leaks. Make sure wires, cap and rotor are in good shape and that the plugs are fresh.
Once timing/ignition is set and verified, the only thing that will cause backfire up through the carb is a lean condition. This will be noticable on acceleration, when a rich mixture is required. If you're popping up through the carb after you've verified the ignition is good, richen up the carb. You'd be much better off getting a correct Q-Jet for the car than to run that old Carter AFB on there...
Lars
Once timing/ignition is set and verified, the only thing that will cause backfire up through the carb is a lean condition. This will be noticable on acceleration, when a rich mixture is required. If you're popping up through the carb after you've verified the ignition is good, richen up the carb. You'd be much better off getting a correct Q-Jet for the car than to run that old Carter AFB on there...
Lars
Drifting
Joined: Dec 2004
Posts: 1,714
Likes: 42
From: Austin Texas
I would check the accelerator pump shot. A lean condition can cause a backfire and you are getting the back fireupon initial acceleration.
If the pump is functioning, it may be that the amount of fuel delivered is not enough.
EDIT - what he said ^.
If the pump is functioning, it may be that the amount of fuel delivered is not enough.
EDIT - what he said ^.
Last edited by johnt365; Nov 12, 2012 at 12:00 PM.
Melting Slicks
Joined: Aug 2010
Posts: 2,487
Likes: 10
From: Palm Beach Florida
St. Jude Donor '12
Before doing any carb work, make sure you check the timing and ignition system. As I state in my tuning seminars, "90% of all carb problems are ignition problems." Get a timing light and set it up per my "How to Set Timing" paper with 36 degrees total advance and initial timing in the 8 - 18 degree range to produce the 36 total. Make sure vacuum advance is hooked up and operational, and make sure there are no vacuum leaks. Make sure wires, cap and rotor are in good shape and that the plugs are fresh.
Once timing/ignition is set and verified, the only thing that will cause backfire up through the carb is a lean condition. This will be noticable on acceleration, when a rich mixture is required. If you're popping up through the carb after you've verified the ignition is good, richen up the carb. You'd be much better off getting a correct Q-Jet for the car than to run that old Carter AFB on there...
Lars
Once timing/ignition is set and verified, the only thing that will cause backfire up through the carb is a lean condition. This will be noticable on acceleration, when a rich mixture is required. If you're popping up through the carb after you've verified the ignition is good, richen up the carb. You'd be much better off getting a correct Q-Jet for the car than to run that old Carter AFB on there...
Lars
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Tech Contributor
Joined: Aug 1999
Posts: 14,377
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From: At my Bar drinking and wrenching in Lafayette Colorado
Lars
Melting Slicks
Joined: Sep 2004
Posts: 2,655
Likes: 19
With oil reformulation, I've seen more than a few boats with partially flat lobed cams (high spring pressures). Idle fine, but spit back through the carb under load when the partially open exhaust valve cant clear the cylinder. Even firing voltages across the cylinders was the same at idle.
It's easier to catch a partial flat lobe on a boat since they're either idle or load. I suspect you can drive a car under light load much longer and finish off the lobe and mushroom the lifter though.
Anyway, thought I would throw that out there after checking the ignition and carb/fuel. But if there was a recent cam change, checking lobes would be high on my list.
It's easier to catch a partial flat lobe on a boat since they're either idle or load. I suspect you can drive a car under light load much longer and finish off the lobe and mushroom the lifter though.
Anyway, thought I would throw that out there after checking the ignition and carb/fuel. But if there was a recent cam change, checking lobes would be high on my list.
I was thinking about that, too, but a flat exhaust lobe will make the engine pop up through the carb even at idle. The fact that the issue only occurs on acceleration would, hopefully, indicate that the cam is still in good shape and that it's more of a tuning and setup issue.
Lars
Lars
Intermediate
Joined: Oct 2008
Posts: 33
Likes: 0
From: Norfolk Virginia
had the same problem turned out to be the cam, my vette sat for 10 years found 3 bad lifters and 2 bad cam lobes, she runs like a champ now, i ordered the cam from summit i told the sales rep i wanted the largest cam available fo rthe the stock valve springs and valves
1st Gear
Joined: Oct 2017
Posts: 1
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Before doing any carb work, make sure you check the timing and ignition system. As I state in my tuning seminars, "90% of all carb problems are ignition problems." Get a timing light and set it up per my "How to Set Timing" paper with 36 degrees total advance and initial timing in the 8 - 18 degree range to produce the 36 total. Make sure vacuum advance is hooked up and operational, and make sure there are no vacuum leaks. Make sure wires, cap and rotor are in good shape and that the plugs are fresh.
Once timing/ignition is set and verified, the only thing that will cause backfire up through the carb is a lean condition. This will be noticable on acceleration, when a rich mixture is required. If you're popping up through the carb after you've verified the ignition is good, richen up the carb. You'd be much better off getting a correct Q-Jet for the car than to run that old Carter AFB on there...
Lars
Once timing/ignition is set and verified, the only thing that will cause backfire up through the carb is a lean condition. This will be noticable on acceleration, when a rich mixture is required. If you're popping up through the carb after you've verified the ignition is good, richen up the carb. You'd be much better off getting a correct Q-Jet for the car than to run that old Carter AFB on there...
Lars
Pro
Joined: Sep 2017
Posts: 516
Likes: 72
From: Modesto California
I have a 1977 vette that I inherited from my uncle. It had been sitting for several years, but I have been working on it, and I have gotten it up and running. It cranks right up and idles fine, but once I press down on the gas it backfires through the carb. Once I put it in gear and go down the road, it will drive fine until I try to accelerate, and then it will backfire through the carb again. It doesn't matter if the engine is warm or cold.
I think that the problem started with my uncle when he did a carb and intake swap on the car. He installed an Edelbrock carb and intake. Is this a timing/fuel issue?? I would appreciate any help. This is driving me crazy!
I think that the problem started with my uncle when he did a carb and intake swap on the car. He installed an Edelbrock carb and intake. Is this a timing/fuel issue?? I would appreciate any help. This is driving me crazy!
There's only two common things that can cause a backfire thru the carburetor and that's excessively early ignition timing and an exhaust valve not opening.
Tech Contributor
Joined: Aug 1999
Posts: 14,377
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From: At my Bar drinking and wrenching in Lafayette Colorado
Actually, the most common cause of backfire through the carb is a lean condition caused by a failed accelerator pump. Lack of pump shot upon sudden opening of the throttle blades will cause an immediate backfire through the carb. Failed pumps are common with the modern pump gas. The fact that the car had been sitting for several years is further indication that the accelerator pump has shriveled up and failed. But ignition should always be checked first. But let's hope that he got it figured out over the past 5 years since he posted the question...
Last edited by lars; Oct 21, 2017 at 10:21 AM.