Backfire thru Exhaust
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Joined: May 2008
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From: Port Washington Wi
Backfire thru Exhaust
I have a 79 with a Qjet . I got it tuned earlier this summer and since then the exhaust has started popping when started cold. It has gotten worse since then. Hot or cold it will pop from an idle until I get into 2nd and around 2K rpm. I've read other posts that indicate the carb is too lean or the timing is off. The car idles great, runs great at wot, so wouldn't this be a too rich at idle problem?
Burning Brakes
Joined: May 2009
Posts: 1,232
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From: Tucson AZ
St. Jude Donor '12
Popping how? (Don't laugh) is it little "poomps" that also feel like misses, or big gunshot "Kapow!" that feels like it should filet the mufflers?
The RPM range sounds like a partially obstructed primary jet.
The RPM range sounds like a partially obstructed primary jet.
Team Owner
Joined: Jul 2008
Posts: 23,419
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From: In limbo
St. Jude Donor '13-'15, '17 thru '22
My first thought is that you are probably a little retarded.
In all seriousness, the first thing I would do is put a timing light on it and see what you get.
In all seriousness, the first thing I would do is put a timing light on it and see what you get.
Race Director
Joined: Dec 2004
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Thread Starter
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Joined: May 2008
Posts: 50
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From: Port Washington Wi
I checked the timing and it was right on. I then advanced it a bit to no effect. Seems to happen only between 1200 - 1500. Checked the primaries and they are clean. I'll try leaning it out a little.
Melting Slicks
Joined: Jul 2006
Posts: 2,277
Likes: 483
From: Salmon Arm, BC
If while idling ,and the car is misfiring,go to the back and smell the exhaust fumes. If there is a strong "gassey" smell the carb is likley set too lean. The gas smell is unburned fuel from incomplete combustion, due to lean conditions. If it is, go turn the mixture screws out one turn exactly each. Go to the back end again and check the smell and see if the engine has smoothed out.
If it was lean, the engine should have smoothed out,or at least be better than before. A couple more adjustments may be necessary.
If it was lean, the engine should have smoothed out,or at least be better than before. A couple more adjustments may be necessary.
Instructor
Joined: May 2009
Posts: 159
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From: tamarac fl
If while idling ,and the car is misfiring,go to the back and smell the exhaust fumes. If there is a strong "gassey" smell the carb is likley set too lean. The gas smell is unburned fuel from incomplete combustion, due to lean conditions. If it is, go turn the mixture screws out one turn exactly each. Go to the back end again and check the smell and see if the engine has smoothed out.
If it was lean, the engine should have smoothed out,or at least be better than before. A couple more adjustments may be necessary.
If it was lean, the engine should have smoothed out,or at least be better than before. A couple more adjustments may be necessary.
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
Of course, as has been suggested, a vacuum leak is the more likely cause of your lean-out vs. idle-mix settings. Our Vettes have an endless array of vacuum hoses. I would start at the carb and work out.
Racer
Joined: Aug 2009
Posts: 310
Likes: 0
From: Oklahoma City OK
I had a pop not too long ago and car died. I was cruising down the highway about 60 when it happened. It was my ignition coil going out. Probably not what your problem is but thought I would share.
