Back fires with down shigting
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Joined: Oct 2002
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From: wellington fl
Back fires with down shigting
I have a 1977 Corvette with a create motor with about 29,000 miles on it. The cars runs great. I have just put new side exhaust on it. It is a 4 speed transmission. When I down shift it backfires. It this normal or is something wrong? The only thing that I have done to the engine since I have had the car is new carb, new plugs and wires and K&N filter. Any suggestions. Thanks
Le Mans Master
Joined: Jan 2000
Posts: 8,721
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From: Canada
check to make sure that your new exhaust has no leaks in it - when I put new headers on mine a number of yrs. ago I had the same issue but found that there was a leak at the collectors - bad weld.
Team Owner
Joined: Jul 1999
Posts: 26,545
Likes: 46
From: Huntersville NC
I 2nd gdh's comment. You just changed the exhaust. An exhaust leak will cause backfiring on downshift (when engine rpm increase without load) Since you just changed the exhaust, a leak is the most likely suspect in my book and the first thing to check.
Burning Brakes
Joined: Oct 2009
Posts: 782
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From: Magic Valley ID
I guess the question that should really be asked, is, WHEN did the backfiring start? Did it start AFTER the carb was put on? And did you put the new stuff on all at the same time? I do agree that the exhaust could have a leak and cause some issues, but if you are getting a considerable "BANG" or "POP" from the exhaust, then I tend to think that this is a fuel (first) or timing (second) issue. If the carb is running too rich, some unburned fuel will heat up and ignite...hence the backfire. Just mt .02. 
Deja
Deja
Drifting
Joined: May 2007
Posts: 1,491
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From: North Royaton Ohio
Is it backfiring or just crackling out the exhaust? What kind of exhaust did you have on it before? I ran a true dual chambered exhaust for a while, and it was more or less muffler-less, and when the RPM's would fall, it would have a little popping out the back. From what I understood it wasn't really a big issue. I actually kind of liked it.
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From: Millington Illinois
St. Jude Donor '08-'09
Reduced backpressure from a new exhaust will cause popping out of the exhaust even with no leaks. Stock timing curve is set up for stock exhaust backpressure and emissions. Check compression to rule out a burnt exhaust valve and adjust timing curve according to the sticky at the top of the page.
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St. Jude Donor '09
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Joined: Feb 2005
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From: coral gables miami
I would suggest that the new carb is on the rich side and excess fuel is collecting in the pipes and burned off with the downshifts causing the noise you hear......your plugs could tell you of the engines condition and if this is correct.....a slight timing advance may help clear it up and then reset the idle back to normal.