[C2] Carb backfire
Thread Starter
Intermediate
Joined: Nov 2022
Posts: 43
Likes: 1
Carb backfire
Hi,
I have a 65l78, re did the front and back fuel bowl gaskets carb, new spark plugs, battery, alternator, and voltage reg and started it up and everything checked out. I took the car out and at one stop after “opening it up” the car was idling at 500 rpms from what I saw and was barely holding on. I managed to keep it alive to drive back to the garage. (Blew some reds). Eventually it would die. Called Holley and they suggested I rebuild everything which I did. Now the car backfires white smoke. I can get it started and with gas keep it alive if I want too but it will die eventually. It’s also cold where I am and I’m outside if that makes a difference. Advice from the pros….. please
I have a 65l78, re did the front and back fuel bowl gaskets carb, new spark plugs, battery, alternator, and voltage reg and started it up and everything checked out. I took the car out and at one stop after “opening it up” the car was idling at 500 rpms from what I saw and was barely holding on. I managed to keep it alive to drive back to the garage. (Blew some reds). Eventually it would die. Called Holley and they suggested I rebuild everything which I did. Now the car backfires white smoke. I can get it started and with gas keep it alive if I want too but it will die eventually. It’s also cold where I am and I’m outside if that makes a difference. Advice from the pros….. please
Thread Starter
Intermediate
Joined: Nov 2022
Posts: 43
Likes: 1
It won’t start, just backfire, if I try buy pumping gas it will start but I would only do that if I was stuck somewhere. The choke is open when I start. The accelerator pump is working fine. There was a leakage problem before I did all the gaskets.
Instructor
Joined: Nov 2006
Posts: 209
Likes: 38
From: Gettysburg
This may seem a bit obvious, but check the gasket that sits between the carb and the intake. The modern gasket I recently installed did not fully cover the "flame trench" (I do not know what else to call the recessed area that is cast into the intake and runs horizontally across the front of the carb.) Once I replaced that with a correct fitting gasket, all problems of backfiring, poor idle, and breaking up while running ceased. I know I will probably take some snark about confessing to this, but but it really was easy not to notice the lack of seal between the carb/intake. If it saves ya some further problems, I am willing to admit such a simple mistake.
Team Owner
Joined: Feb 2003
Posts: 54,073
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From: Sitting in his Nowhere land Hanover Pa
2024 C2 of the Year Finalist - Unmodified
2015 C2 of Year Finalist
This may seem a bit obvious, but check the gasket that sits between the carb and the intake. The modern gasket I recently installed did not fully cover the "flame trench" (I do not know what else to call the recessed area that is cast into the intake and runs horizontally across the front of the carb.) Once I replaced that with a correct fitting gasket, all problems of backfiring, poor idle, and breaking up while running ceased. I know I will probably take some snark about confessing to this, but but it really was easy not to notice the lack of seal between the carb/intake. If it saves ya some further problems, I am willing to admit such a simple mistake.
Le Mans Master
Joined: Jan 2007
Posts: 5,996
Likes: 1,502
From: Prescott Arizona
I thought that SS baffle/plate was only used on the 250/300 horse intakes, to cover over the heat crossover passage? Didn't think the factory 396 intake used it but believe it showed up later, like on the 69 390 427s.
Mike T - Prescott AZ
Mike T - Prescott AZ
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Team Owner
Joined: Feb 2003
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From: Sitting in his Nowhere land Hanover Pa
2024 C2 of the Year Finalist - Unmodified
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I was asking pervette that as I know he has a 300. But that does not relate to the OPs engine if he is using a factory al intake
Le Mans Master
Joined: Jan 2007
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From: Prescott Arizona
Thread Starter
Intermediate
Joined: Nov 2022
Posts: 43
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It was running fine but was slow with acceleration. I first changed the fuel bowl gaskets and washers first, started it up and all was good. No leaks let it sit there for 10 min. Took it to get gas added one bottle of VP racing and 93 octane and after I started it something seemed off. Vacuumed the interior of the car and started it again, Seemed Fine. I got to a light and gunned it since it was running way better power wise then before. At my office it was barely holding on when I parked it and my gauge was reading 500 rpm. Kept it alive by giving gas, when I gave gas it didn’t die. Was able to drive it a km away where my garage is. Has not started without me pumping ever since. If I pump it will start but I need to keep it alive it won’t idle with me gassing it. The backfire makes me pretty nervous.
Last edited by 65L78; Dec 7, 2022 at 06:34 PM.
Race Director
Joined: Nov 2003
Posts: 10,696
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From: Cottonwood AZ
C1 of Year Finalist (performance mods) 2019
I don’t know if this will effect your problem but if your carb has backfired since you rebuilt it I would check the power valve. It does not take much of a backfire to ruin one of them.
Skip Shift
Joined: Jan 2007
Posts: 2,031
Likes: 419
From: Citrus Hills, Florida
Don't know where you are located. Maybe water in gas as you stated was running ok until you got fuel, put in VP additive etc. Might want to run some fuel into a container and see if has water. Everything else make little sense as far as the carb repair goes. Especially if it was running fine prior to the fuel fill. - Fred
Le Mans Master
Joined: Nov 2015
Posts: 7,444
Likes: 2,625
From: Really Central IL Illinois
Most likely vacuum leak at the baseplate, but could be power valve especially if it was changed and the wrong gasket was installed. The 3 hole power valve takes the triangular gasket and the 2 slot power valve takes the round hole gasket if I recall right. Also, backfiring could be running lean as is indicated by the rpm loss. Fuel pump, cracked hose, hole in fuel line, stuck float, etc., etc.,etc. Go to the Holley site for some help also. https://www.holley.com/support/carburetor/
If you don't have a vacuum gauge, I highly recommend you get one or use the one on your mighty vac or other tools.
As stated before, check the timing, dwell, spark plug wire routing to correct plug, etc. Basically verify every thing you did again to find what you did wrong.
If you don't have a vacuum gauge, I highly recommend you get one or use the one on your mighty vac or other tools.
As stated before, check the timing, dwell, spark plug wire routing to correct plug, etc. Basically verify every thing you did again to find what you did wrong.
Pro
Joined: Feb 2015
Posts: 625
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From: Agawam MA.
C2 of Year Finalist (appearance mods) 2019
If I've interpreted the phrase, " after opening it up " correctly in your post. Before I second guessed all my previous work , I'd remove the rocker covers an take a look .
It's been my experience that lots of bad things can happen , after opening it up!
Hope I'm wrong, good luck
It's been my experience that lots of bad things can happen , after opening it up!
Hope I'm wrong, good luck
Le Mans Master
Joined: Jan 2007
Posts: 5,996
Likes: 1,502
From: Prescott Arizona
65L78 - Not too long ago, I found myself pulling the carb on my 66 L79 and freshening up a few things and that meant adding new bowl and metering plates gaskets. I like to use the Holley Blue Reusable type gaskets.
I always thought they would seal better than the harder/thinner brown paper gaskets. For quite a few miles afterwards, the car ran fine, no problems. Then, one early Saturday morning, on my way back from the 'Car-Guy-Get-Together' down at the Donut Shop,
the car started to balk, almost like it was about to die. Oddly, it didn't die but ran really ragged. Here's the thread I wrote on the subject. Reason for posting this is due to what the actual culprit was and in my case...it was the new Blue Holley Gaskets.
Check out post #25 on this thread and notice the tiny Blue shard of gasket material that got stuck in the idle feed line.
Here's the thread: C2 Engine Running Ragged - Suspect Pertronix or Ignition Switch? - CorvetteForum - Chevrolet Corvette Forum Discussion
Mike T - Prescott AZ
I always thought they would seal better than the harder/thinner brown paper gaskets. For quite a few miles afterwards, the car ran fine, no problems. Then, one early Saturday morning, on my way back from the 'Car-Guy-Get-Together' down at the Donut Shop,
the car started to balk, almost like it was about to die. Oddly, it didn't die but ran really ragged. Here's the thread I wrote on the subject. Reason for posting this is due to what the actual culprit was and in my case...it was the new Blue Holley Gaskets.
Check out post #25 on this thread and notice the tiny Blue shard of gasket material that got stuck in the idle feed line.
Here's the thread: C2 Engine Running Ragged - Suspect Pertronix or Ignition Switch? - CorvetteForum - Chevrolet Corvette Forum Discussion
Mike T - Prescott AZ