C3 backfire.
Thread Starter
Cruising
Joined: Dec 2017
Posts: 13
Likes: 0
C3 backfire.
Hello all, I've run into a bit of my problem with my Corvette.
I bought the car about a year ago, ran and drove just fine. It's a '76 L82 4 speed Corvette. I drove it off the trailer at the house, and put it in the corner and it sat until about 2 weeks ago. Again, fired right up and put it up on the lift.
Over the last two weeks I put a new intake and carb on (Edelbrock 1406) did side pipes, drained the fuel, all new fuel lines and pump, changed the oil, new plugs, wires, and did cap,rotor,coil,exc for the distributor.
It fired right up and runs and drives, but I have a loud exhaust backfire on initial acceleration. Carb seems to be fairly close (plugs look okay...) and i'm around 36 degrees total at 2700RPM. Running manifold vacuum to Dist.
Speaking of manifold vacuum, my gauge is very erratic at all RPM. unreadable. My passenger side exhaust ( side the backfire is coming from) is also noticeably quieter than the drivers side.
Any help would be nice, I'm hoping its not valves. Thanks!
I bought the car about a year ago, ran and drove just fine. It's a '76 L82 4 speed Corvette. I drove it off the trailer at the house, and put it in the corner and it sat until about 2 weeks ago. Again, fired right up and put it up on the lift.
Over the last two weeks I put a new intake and carb on (Edelbrock 1406) did side pipes, drained the fuel, all new fuel lines and pump, changed the oil, new plugs, wires, and did cap,rotor,coil,exc for the distributor.
It fired right up and runs and drives, but I have a loud exhaust backfire on initial acceleration. Carb seems to be fairly close (plugs look okay...) and i'm around 36 degrees total at 2700RPM. Running manifold vacuum to Dist.
Speaking of manifold vacuum, my gauge is very erratic at all RPM. unreadable. My passenger side exhaust ( side the backfire is coming from) is also noticeably quieter than the drivers side.
Any help would be nice, I'm hoping its not valves. Thanks!
Le Mans Master
Joined: Mar 2004
Posts: 7,458
Likes: 1,484
From: Little Rock AR
Backfire is through carb. Afterfire is out the exhaust.
Vacuum leak is possible. Double check firing order on the wires and look for a spark plug wire burning on the exhaust. A crossed wire can cause some problems. Carb needs to be adjusted properly to work right.
Its not the valves.
Vacuum leak is possible. Double check firing order on the wires and look for a spark plug wire burning on the exhaust. A crossed wire can cause some problems. Carb needs to be adjusted properly to work right.
Its not the valves.
Racer
Joined: Nov 2017
Posts: 289
Likes: 32
Hello all, I've run into a bit of my problem with my Corvette.
I bought the car about a year ago, ran and drove just fine. It's a '76 L82 4 speed Corvette. I drove it off the trailer at the house, and put it in the corner and it sat until about 2 weeks ago. Again, fired right up and put it up on the lift.
Over the last two weeks I put a new intake and carb on (Edelbrock 1406) did side pipes, drained the fuel, all new fuel lines and pump, changed the oil, new plugs, wires, and did cap,rotor,coil,exc for the distributor.
It fired right up and runs and drives, but I have a loud exhaust backfire on initial acceleration. Carb seems to be fairly close (plugs look okay...) and i'm around 36 degrees total at 2700RPM. Running manifold vacuum to Dist.
Speaking of manifold vacuum, my gauge is very erratic at all RPM. unreadable. My passenger side exhaust ( side the backfire is coming from) is also noticeably quieter than the drivers side.
Any help would be nice, I'm hoping its not valves. Thanks!
I bought the car about a year ago, ran and drove just fine. It's a '76 L82 4 speed Corvette. I drove it off the trailer at the house, and put it in the corner and it sat until about 2 weeks ago. Again, fired right up and put it up on the lift.
Over the last two weeks I put a new intake and carb on (Edelbrock 1406) did side pipes, drained the fuel, all new fuel lines and pump, changed the oil, new plugs, wires, and did cap,rotor,coil,exc for the distributor.
It fired right up and runs and drives, but I have a loud exhaust backfire on initial acceleration. Carb seems to be fairly close (plugs look okay...) and i'm around 36 degrees total at 2700RPM. Running manifold vacuum to Dist.
Speaking of manifold vacuum, my gauge is very erratic at all RPM. unreadable. My passenger side exhaust ( side the backfire is coming from) is also noticeably quieter than the drivers side.
Any help would be nice, I'm hoping its not valves. Thanks!
An erratic vacuum gauge reading indicates a problem with the open and closing of the valves. Do a compression check and I'll bet you'll find a very low cylinder pressure on at least one cylinder. You may have a flat camshaft lobe or a problem with a push rod/rocker arm on one of the valves.
Last edited by NeverTooOld; Dec 27, 2017 at 04:34 PM.
Thread Starter
Cruising
Joined: Dec 2017
Posts: 13
Likes: 0
An erratic vacuum gauge reading indicates a problem with the open and closing of the valves. Do a compression check and I'll bet you'll find a very low cylinder pressure on at least one cylinder. You may have a flat camshaft lobe or a problem with a push rod/rocker arm on one of the valves.
Did compression check- Cyl 6 has 1-2 PSI of compression. All others are 120ish.
Looks like I'm doing heads... Any bolt-on recommendations that go well with the L82 cam and Edelbrock performer with stock forged pistons?
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Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Compression that low, either the valves are not doing their job or something in the valve train has died. If you don't find anything there, THEN it's time to pull the head.
Thread Starter
Cruising
Joined: Dec 2017
Posts: 13
Likes: 0
Did this- cyl. 6 exhaust valve spring is very loose- was able to push it down about 1/4 inch by hand. May be just a broke spring and i'm able to pull it. Everything else seemed to be in good shape and I'm not able to move any others.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,809
From: Cool Northern Michigan
Do you have or can you borrow a valvespring compresser tool that fits on the rocker stud? Now you know why the erratic vac gauge needle, backfire, low compression.
So now what? All 16 new springs?
So now what? All 16 new springs?
Thread Starter
Cruising
Joined: Dec 2017
Posts: 13
Likes: 0
I don't, but I'm sure I can find one, or build one. I've never been this far into a Chevy, I come from the Mopar side of the world. Could I replace my bad spring and be on my way? Or should I do all 16? Or pull head and make sure I still have something resembling a valve seat? Heads are bone stock L82's.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,809
From: Cool Northern Michigan
I don't want to make the decision for you without a visual. Cheap way out would be new spring, old retainer. It might be the splitlocks gave way. If you replace one spring, it should match the pressure ratings of the other 15 fairly close.
The spring compresser tool you need is around $50 from Summit. Then you will need a way to hold the valve up in the head. Piece of rope thru the plug hole works as good as compressed air.
You will have a better idea of whats going on when the spring is removed.
The spring compresser tool you need is around $50 from Summit. Then you will need a way to hold the valve up in the head. Piece of rope thru the plug hole works as good as compressed air.
You will have a better idea of whats going on when the spring is removed.
Last edited by HeadsU.P.; Dec 28, 2017 at 11:08 AM.
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
This tool:
https://www.summitracing.com/int/par...-proform-parts $20
And a way to either put and keep air in the cylinder or a piece of rope. Hold the valve up while you remove the locks and retainer and be sure to not drop it. Not tough at all.
https://www.summitracing.com/int/par...-proform-parts $20
And a way to either put and keep air in the cylinder or a piece of rope. Hold the valve up while you remove the locks and retainer and be sure to not drop it. Not tough at all.
Last edited by TimAT; Dec 27, 2017 at 09:06 PM.
Thread Starter
Cruising
Joined: Dec 2017
Posts: 13
Likes: 0
Thanks everyone, I will find the parts I need and go from there. depending on what I find I may do all 16. In the morning I will pull the spring and make sure that is my problem.
Again, this has been a big help! Thanks for the response, I really appreciate it!
Again, this has been a big help! Thanks for the response, I really appreciate it!
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,809
From: Cool Northern Michigan
If you intend to do all 16 springs, get the better tool and order new stem seals. If you are just doing the one spring the cheaper tool will suffice.
Watch your rocker arms while jogging the IGN key. As the piston nears TDC on "that" cyl and both valves close, shove some soft pliable rope in the sparkplug hole. Then theres no way the valve can drop too far. Use a magnet to remove your splitlocks. Cover / block all oil drain holes in the head with rags. Inspect the spring, retainer, stem seal and splitlocks closely for damage.
Watch your rocker arms while jogging the IGN key. As the piston nears TDC on "that" cyl and both valves close, shove some soft pliable rope in the sparkplug hole. Then theres no way the valve can drop too far. Use a magnet to remove your splitlocks. Cover / block all oil drain holes in the head with rags. Inspect the spring, retainer, stem seal and splitlocks closely for damage.
Thread Starter
Cruising
Joined: Dec 2017
Posts: 13
Likes: 0
If you intend to do all 16 springs, get the better tool and order new stem seals. If you are just doing the one spring the cheaper tool will suffice.
Watch your rocker arms while jogging the IGN key. As the piston nears TDC on "that" cyl and both valves close, shove some soft pliable rope in the sparkplug hole. Then theres no way the valve can drop too far. Use a magnet to remove your splitlocks. Cover / block all oil drain holes in the head with rags. Inspect the spring, retainer, stem seal and splitlocks closely for damage.
Watch your rocker arms while jogging the IGN key. As the piston nears TDC on "that" cyl and both valves close, shove some soft pliable rope in the sparkplug hole. Then theres no way the valve can drop too far. Use a magnet to remove your splitlocks. Cover / block all oil drain holes in the head with rags. Inspect the spring, retainer, stem seal and splitlocks closely for damage.
Will do, I was debating the rope trick or the compressed air trick.Nothing was coming up in a base search and I was trying to find some parts- does anyone know the valve seat pressure on a bone stock L82? I'm thinking it's probably somewhere around 80, right?