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Help! Backfire thru Carb

rallemandi

Member
Joined
Feb 27, 2007
Messages
11
Location
Aptos, CA
Corvette
1981 Coupe
Hi - I have a totally stock 81 with a 350 and an auto trans. One day, it started backfiring through the carb. Usually I would be inclined to think something like that would be timing related, but the timing on the 81 is all electronic through the computer. I've verified that all the spark plug wires are in the right spot. I've checked for vaccum leaks and found that the ported vaccum sensors out of the thermostat housing were both broken. I replaced them both, but that didn't fix anything. Any suggestions?

Thanks!
 
Hi - I have a totally stock 81 with a 350 and an auto trans. One day, it started backfiring through the carb. Usually I would be inclined to think something like that would be timing related, but the timing on the 81 is all electronic through the computer. I've verified that all the spark plug wires are in the right spot. I've checked for vaccum leaks and found that the ported vaccum sensors out of the thermostat housing were both broken. I replaced them both, but that didn't fix anything. Any suggestions?

Thanks!
How many Miles are on the car??When does it Backfire?? Hows the accelerator pump in the carb?? How loose is the timing chain?? Is there a chunk of carbon in the EGR valve??Does the distributer cap have a crack or moisture in it??Is the coil arcing?? Is the base plate gasket on the carb sucking air??Is the fuel pump delivering enough fuel?? Any of these can cause a Back fire!! We need more Info!!:upthumbs
 
Timing. Someone can chime in here, but I'm not certain the '81 computer controls timing. Most of what it does is emissions related.

:)
 
How many Miles are on the car??When does it Backfire?? Hows the accelerator pump in the carb?? How loose is the timing chain?? Is there a chunk of carbon in the EGR valve??Does the distributer cap have a crack or moisture in it??Is the coil arcing?? Is the base plate gasket on the carb sucking air??Is the fuel pump delivering enough fuel?? Any of these can cause a Back fire!! We need more Info!!:upthumbs

The car only has 74,000 original miles. It backfires only under hard acceleration. The carb is clean and doesn't suck air at the base. The dist cap, and rotor are brand new. The timing chain is tight. I have plenty of fuel. When I had it smogged about a year ago, they told me the egr valve was stuck open but not to worry about it. They said it was better to be stuck open then closed. Do you think that could be my problem?

Thanks,
Rick
 
If the EGR is stuck open, it will foul the plugs and cause idle problems or possibly misfire under acceleration.
Another thing that would cause backfire under acceleration is a bad timing advance module, since the computer controls the advance.
Or possibly a weak intake valve spring or sticky intake valve guide.
 
Timing. Someone can chime in here, but I'm not certain the '81 computer controls timing. Most of what it does is emissions related.

:)
Yep. On the 81 the timing is controlled electronically by a spark advance module inside the distributor.
 
One other thing to check is the MAP sensor that tells the ECM what your manifold vaccuum is.
 
I had the same problem with my '79. About the same mileage. Replaced everything I knew including the carb. Would idle perfectly. Had plenty of fuel and the right pressure too. As soon as I tried to goose the throttle, it would sputter and backfire thru the carb. It turned out to be worn lobes on the cam at the #4 and #6 cylinder. Had the cam , lifters, and pushrods checked and/or replaced and we are fine now. Hope it's simpler (and cheaper) for you. Good luck.........
 
sounds like possible worn cam to me also Steve
 
I had a '69 SS-396 Chevelle with the same symptoms. Turned out to be 2 broken intake valve springs. Replaced the springs and no more backfiring thru the carb.
Andy
 
Or possibly a weak intake valve spring or sticky intake valve guide.

:) I agree :):):beer
 
I would suspect timing. 74,000 miles on the original timing chain sounds about right. I would guess the nylon teeth or worn, or the chain stretched.

It happened to my '82. I could get it to idle, but it ran like a dog and backfired. I spent 3 weeks checking everything hoping it wasn't the timing chain. I finally gave up and replaced the timing chain. The old one was stretched almost 1 inch and had jumped a few teeth. On the bright side, i got to replace a lot of worn out parts as i dug my way to the timing chain.

I hope it is something simpler.

Good luck!
 

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