Carberator backfire
Thread Starter
Cruising
Joined: Nov 2012
Posts: 13
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From: Sparks NV
Carberator backfire
My 1980 C3 350 is not running correctly. When I floor it to go into passing gear, it backfires through the carburetor until I leave off the gas. Other than that it seems to run ok. A mechanic said (without doing anything) that it has a flat cam, and it will cost $1,400 to replace. Is there anything other than that which could be causing this?
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Joined: Jun 2013
Posts: 78
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From: New Holland Pa
My 1980 C3 350 is not running correctly. When I floor it to go into passing gear, it backfires through the carburetor until I leave off the gas. Other than that it seems to run ok. A mechanic said (without doing anything) that it has a flat cam, and it will cost $1,400 to replace. Is there anything other than that which could be causing this?
I recently bought mine and it was like that. As long as I babied it or got into it slowly it was OK. After some investigating, I found by mechanical advance wasn't working. Now it almost make me giggle when I stomp it!
Tech Contributor
Joined: Aug 1999
Posts: 14,379
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From: At my Bar drinking and wrenching in Lafayette Colorado
If you have a flat cam, it won't be running on all 8 cylinders - it will have a miss. Do you have a miss..?
If not, your secondary airvalve spring windup is set too loose. Set the spring windup to 3/4 turn and it will transition into the secondaries just fine with no backfire.
Lars
If not, your secondary airvalve spring windup is set too loose. Set the spring windup to 3/4 turn and it will transition into the secondaries just fine with no backfire.
Lars
Last edited by lars; Nov 10, 2013 at 05:40 PM.
Burning Brakes
Joined: Sep 2012
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From: coon rapids mn
do whats lars says .also check your cap and rotor ,change them if the contacts don't look good.maybe you have a valve not seating right .maybe do a leak down test .check for bent push rods also.
Le Mans Master
Joined: Jun 2004
Posts: 6,231
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From: Seattle WA
St. Jude Donor '14
I'm a fan of making sure the timing is right before moving on to the carb. It just provides a solid foundation where you know one thing is right...so any changes you make moving forward - one at a time - should show improvement.
Le Mans Master
Joined: Mar 2008
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From: Oxford MA-----You just lost the game!!!!
Yes, there is an old saying: "90% of carb problems are actually ignition problems." So, it's worth the time and effort to make sure your ignition is up to snuff before moving on to the carb.
Actually, try Lars's suggestion first. He's The Man, and I am certainly not going to contradict Him. Plus, it's real easy to do. You might want to think about sending Lars an email, and asking nicely for a copy of his papers on How To Set Timing, and How To Tune A Q-jet. They are free, and they come with a double-your-money-back guarantee!
Scott
Actually, try Lars's suggestion first. He's The Man, and I am certainly not going to contradict Him. Plus, it's real easy to do. You might want to think about sending Lars an email, and asking nicely for a copy of his papers on How To Set Timing, and How To Tune A Q-jet. They are free, and they come with a double-your-money-back guarantee!
Scott
Racer
Joined: Oct 2013
Posts: 254
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From: Tampa Bay Florida
My 1980 C3 350 is not running correctly. When I floor it to go into passing gear, it backfires through the carburetor until I leave off the gas. Other than that it seems to run ok. A mechanic said (without doing anything) that it has a flat cam, and it will cost $1,400 to replace. Is there anything other than that which could be causing this?
First of all, bypass that mechanic and run like hell in any direction from his garage. Did he do any tests before coming up with his $1400 cure? A simple manifold vacuum test will tell you if you have a bad cam. You most likely have an under $100 problem. Check all your vac lines for cracks, loose fits or other leaks. Check to see that your accelerator pump is pumping, and if your motor is gagging on a downshift, it could be that your secondaries are on the lean side. A weak fuel pump or clogged fuel filter could also be the source of your headache. But whatever you do, stay away from that guy who wants you to fund his IRA.
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Race Director
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
If it backfireing through the carb the first thing I would look for is a vacuum leak more specifcaly if you were just working on the engine. then a hose might not be connected.
This might not be noticed until you hit the happy pedal and full load on the engine
Next thing to check is your timing correct at 3000rpm
This might not be noticed until you hit the happy pedal and full load on the engine
Next thing to check is your timing correct at 3000rpm
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
I think the point here is that it could be a lot of things. You're going to have to check them all, unless you can diagnose the problem further. I like to start with the easiest/cheapest things first, and then move on from there. Let's start at the beginning: How long have you had this car, and has it always done this, or did it start doing it since you started driving it?
Scott
Scott
Thread Starter
Cruising
Joined: Nov 2012
Posts: 13
Likes: 0
From: Sparks NV
My 1980 C3 350 is not running correctly. When I floor it to go into passing gear, it backfires through the carburetor until I leave off the gas. Other than that it seems to run ok. A mechanic said (without doing anything) that it has a flat cam, and it will cost $1,400 to replace. Is there anything other than that which could be causing this?
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
Had the car about 1 year. It was owned by a man in Florida, but was garaged a long time in Frostburg, VA. His brother up there sold it for him. I drove it through mountains and up to Harrisburg, PA. and shipped it to Reno, NV where I am now. It seems to run fine with just babying it. Just when I hit passing gear (automatic transmission). I have put more than 3 tanks of gas since getting it. So, I don't think it is bad gas?? I want to thank all you guys! I will take notes, and go to another mechanic to get it fixed.
I have a feeling that a good, basic tune-up would have a good chance of solving your drivability problem. At the very least, it would rule out a lot of possible causes. If you are not up to it, find a good mechanic who has a good reputation for working on older cars. If there are any hot rod clubs in your area, they may be able to point you at someone good. Or perhaps someone on this forum who lives in your area could recommend someone. Good luck!
Edit: Corvettes were not made to be babied! Drive it like you stole it once in awhile!
Scott
Last edited by scottyp99; Nov 12, 2013 at 11:10 AM.
Thread Starter
Cruising
Joined: Nov 2012
Posts: 13
Likes: 0
From: Sparks NV
If you have a flat cam, it won't be running on all 8 cylinders - it will have a miss. Do you have a miss..?
If not, your secondary airvalve spring windup is set too loose. Set the spring windup to 3/4 turn and it will transition into the secondaries just fine with no backfire.
Lars
If not, your secondary airvalve spring windup is set too loose. Set the spring windup to 3/4 turn and it will transition into the secondaries just fine with no backfire.
Lars
Thread Starter
Cruising
Joined: Nov 2012
Posts: 13
Likes: 0
From: Sparks NV
Wow, 3 tanks of gas in a year? You're really driving the wheels off that thing, huh?
I have a feeling that a good, basic tune-up would have a good chance of solving your drivability problem. At the very least, it would rule out a lot of possible causes. If you are not up to it, find a good mechanic who has a good reputation for working on older cars. If there are any hot rod clubs in your area, they may be able to point you at someone good. Or perhaps someone on this forum who lives in your area could recommend someone. Good luck!
Edit: Corvettes were not made to be babied! Drive it like you stole it once in awhile!
Scott
I have a feeling that a good, basic tune-up would have a good chance of solving your drivability problem. At the very least, it would rule out a lot of possible causes. If you are not up to it, find a good mechanic who has a good reputation for working on older cars. If there are any hot rod clubs in your area, they may be able to point you at someone good. Or perhaps someone on this forum who lives in your area could recommend someone. Good luck!
Edit: Corvettes were not made to be babied! Drive it like you stole it once in awhile!
Scott
Le Mans Master
Joined: Jun 2004
Posts: 6,231
Likes: 65
From: Seattle WA
St. Jude Donor '14
I think you're going to find that if you want to own a C3 you're going to have to invest in acquiring basic mechanics skills. There's no suggested troubleshooting/repair actions here that can't be accomplished by the average guy with simple, inexpensive tools, a couple of hours, the factory service manual...and a little help from the Forum.
Sadly, most mechanics today don't have the skills or experience to work on "old skool", non-computer-controlled engines. If you decide to take it to a shop, you're better to search for a "hot rod" or aftermarket performance shop - one with 60's muscle cars parked out front.
I recently helped a guy out with a '68 that had been "tuned" by a local shop into being completely undriveable. Not only had they locked in the mechanical advance and disconnected (and not plugged) the vacuum advance, but the crimes they perpetrated on the poor Edelbrock carb were punishable by spending the rest of their lives driving a Prius.
Sadly, most mechanics today don't have the skills or experience to work on "old skool", non-computer-controlled engines. If you decide to take it to a shop, you're better to search for a "hot rod" or aftermarket performance shop - one with 60's muscle cars parked out front.
I recently helped a guy out with a '68 that had been "tuned" by a local shop into being completely undriveable. Not only had they locked in the mechanical advance and disconnected (and not plugged) the vacuum advance, but the crimes they perpetrated on the poor Edelbrock carb were punishable by spending the rest of their lives driving a Prius.
