Backfiring Through Carburetor Help Needed
Thread Starter
Cruising
Joined: Apr 2011
Posts: 14
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Backfiring Through Carburetor Help Needed-Update 7/16!
Hello,
I have a 327/300 hp 1968 Corvette with a Carter AFB Carburetor. It has not been driven much in well over a decade. I replaced the distributor cap, wires, rotor, and replaced a blown exhaust manifold gasket and the car was running great for about a week. Then, at speed, I hear a loud clunking sound under the hood which I determined to be backfiring through the carburetor. I checked the points and all the wires to make sure they were secure and set the idle at 700 with a dwell tach and set the timing to 4 degrees BTDC...now it is backfiring and sputtering even worse than before and the idle is not even steady. I am also blowing a lot of white smoke out of the exhaust. I am stumped and do not know what else I should check. Thank you for any advice.
I have a 327/300 hp 1968 Corvette with a Carter AFB Carburetor. It has not been driven much in well over a decade. I replaced the distributor cap, wires, rotor, and replaced a blown exhaust manifold gasket and the car was running great for about a week. Then, at speed, I hear a loud clunking sound under the hood which I determined to be backfiring through the carburetor. I checked the points and all the wires to make sure they were secure and set the idle at 700 with a dwell tach and set the timing to 4 degrees BTDC...now it is backfiring and sputtering even worse than before and the idle is not even steady. I am also blowing a lot of white smoke out of the exhaust. I am stumped and do not know what else I should check. Thank you for any advice.
Last edited by SFT; Jul 16, 2011 at 10:13 PM.
Le Mans Master
Joined: Mar 2008
Posts: 5,948
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From: Oxford MA-----You just lost the game!!!!
Don't you mean "carbyrator"?
Seriously, tho, doesn't white smoke usually mean coolant in the combustion chamber? Look in that direction, blown head gasket, maybe? Hope not,
Scott
Last edited by scottyp99; Jul 14, 2011 at 02:39 AM.
Pro
Joined: Aug 2010
Posts: 698
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From: Stockholm Sweden
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
Le Mans Master
Joined: Sep 2004
Posts: 5,116
Likes: 5
From: Livonia MI
How old's the fuel in the tank, because it loses octane rating over the years and could be pre-igniting. Something to check for. If you decide to syphon it out, you can always mix a bit in your daily car to get rid of it.
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Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
I am not really sure how, but I have a feeling that the most important clue we have so far is that it ran great for a week, then there was a big, ugly noise, and now, all of a sudden, it runs like crap. have you pulled the spark plugs and inspected them?
Scott
Scott
Burning Brakes
Joined: Jul 2010
Posts: 755
Likes: 8
From: delco pa
have you gotten gas just before it started the problems? could have gotten bad gas then,,,
Pro
Joined: Aug 2010
Posts: 698
Likes: 8
From: Stockholm Sweden
Could also have been fragments of rust in the gastank that clogged the filter/carb after a while. 
Have you checked the bottom of the gastank?
Just light it up with a flashlight and spot trough the filling hole.
One sholud be able to se if there is any collections of water and/or rust in there.
Be careful not to drop the flashlight in the petrol thogh..
//Ricky.
Have you checked the bottom of the gastank?
Just light it up with a flashlight and spot trough the filling hole.
One sholud be able to se if there is any collections of water and/or rust in there.
Be careful not to drop the flashlight in the petrol thogh..
//Ricky.
Thread Starter
Cruising
Joined: Apr 2011
Posts: 14
Likes: 0
thanks for the help so far everyone. I had already used up half a tank of gas before the problems started occurring so I don't think it was bad gasoline. I will look into the other possible problems suggested as soon as possible and let you guys know if I was able to fix it. thanks again
Pro
Joined: Aug 2010
Posts: 698
Likes: 8
From: Stockholm Sweden
thanks for the help so far everyone. I had already used up half a tank of gas before the problems started occurring so I don't think it was bad gasoline. I will look into the other possible problems suggested as soon as possible and let you guys know if I was able to fix it. thanks again
One can not take for granted that one always get clean fuel when filling up. The petrol has passed several different storage containers and trucks before it gets in your Vettes petrol tank.
The tank on the petrol station may be dug down in the ground and there may be condense water in the fuel due to that.
I have experinsed this a couple of times.
I hope you solve the problem.
//Ricky.
Racer
Joined: Mar 2008
Posts: 434
Likes: 0
From: Triangle Area NC
Might be good to drain the tank and see what you find... if it's full of rusty crap, you might need to work your way forward to see if any of the fuel passages (line, pump, filter, carb) are clogged or restricted. One of those clear glass fuel filters temporarily installed might give you an indication of what's coming thru the line.
John
John
Thread Starter
Cruising
Joined: Apr 2011
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Hi everyone,
thanks again for the input. I just finished doing a compression test to see if the problem was inside the engine. Solely based on the PSI results, it would seem as if this wasn't the case, since all of the cylinders had a strong output of between 160 and 190 PSI (factory specifications for my engine is 160). However, when I was testing one cylinder (so this one particular cylinder is plugged with the compression kit while the rest are open), I heard a popping sound which I identified to be backfiring through the carb - seems too coincidental to be insignificant? This backfiring did not occur when any of the other 7 cylinders were being tested. Does anybody know what this signifies? Does that mean I have a sticky valve or some other internal problem even though the compression in that cylinder was strong?
Also, I checked out the fuel tank...it looks very clean to me with virtually no rust or dirt...found a crack in a rear fuel line and am in the process of replacing that.
Please let me know your thoughts. Thanks again
thanks again for the input. I just finished doing a compression test to see if the problem was inside the engine. Solely based on the PSI results, it would seem as if this wasn't the case, since all of the cylinders had a strong output of between 160 and 190 PSI (factory specifications for my engine is 160). However, when I was testing one cylinder (so this one particular cylinder is plugged with the compression kit while the rest are open), I heard a popping sound which I identified to be backfiring through the carb - seems too coincidental to be insignificant? This backfiring did not occur when any of the other 7 cylinders were being tested. Does anybody know what this signifies? Does that mean I have a sticky valve or some other internal problem even though the compression in that cylinder was strong?
Also, I checked out the fuel tank...it looks very clean to me with virtually no rust or dirt...found a crack in a rear fuel line and am in the process of replacing that.
Please let me know your thoughts. Thanks again
Thread Starter
Cruising
Joined: Apr 2011
Posts: 14
Likes: 0
hello again,
I just removed the valve cover and believe I found the source of my problem. One of the push rods is coming out at the wrong angle and so its rocker arm is not connected in the spot where it should be...I tried to simply move the rocker arm back in place but it won't let me. I believe it is because the push rod is coming out at the wrong angle (maybe it is bent? I'm not sure).
Can anyone tell me how serious of a problem this is and how I can go about fixing it? Is it simply adjusting the rod or pulling it and replacing it or does this signify some other problem deeper inside the engine?
The engine ran perfectly for about a week until I was pulling onto the freeway one day and heard a popping sound which I identified to be backfiring through the carb. This must be the cause of my problem.
I tried to attach pics but I couldn't figure out how to so instead I took a youtube video of the problem so you all can see it firsthand....as they say a picture is worth a thousand words.
http://www.youtube.com/watch?v=QE-oShxiPsA
Thanks for the help.
I just removed the valve cover and believe I found the source of my problem. One of the push rods is coming out at the wrong angle and so its rocker arm is not connected in the spot where it should be...I tried to simply move the rocker arm back in place but it won't let me. I believe it is because the push rod is coming out at the wrong angle (maybe it is bent? I'm not sure).
Can anyone tell me how serious of a problem this is and how I can go about fixing it? Is it simply adjusting the rod or pulling it and replacing it or does this signify some other problem deeper inside the engine?
The engine ran perfectly for about a week until I was pulling onto the freeway one day and heard a popping sound which I identified to be backfiring through the carb. This must be the cause of my problem.
I tried to attach pics but I couldn't figure out how to so instead I took a youtube video of the problem so you all can see it firsthand....as they say a picture is worth a thousand words.
http://www.youtube.com/watch?v=QE-oShxiPsA
Thanks for the help.
Instructor
Joined: Jun 2011
Posts: 183
Likes: 1
From: Litchfield CT
This is an easy one. Get a 5/8" socket, and loosen the gland nut on the rocker 1/4 turn at a time. After each 1/4 turn, grab the rocker and twist until you can get the pushrod back in place, and the tip of the rocker back on the valve.
After you get things back to normal, re-tighten the nut the same number of turns yiu loosened it. Then add another 1/4 turn, becasue the rocker was most likely too loose.
After you get things back to normal, re-tighten the nut the same number of turns yiu loosened it. Then add another 1/4 turn, becasue the rocker was most likely too loose.
