backfire on deceleration
Thread Starter
Racer
Joined: Sep 2002
Posts: 293
Likes: 0
From: Englewood CO
backfire on deceleration
I just installed votec heads, a perfomer vortec manifold, headers, and a dual exhaust with targa style mufflers. Now on decleration the car backfires, only once the car has been warmed up though. Also if I rev it and it comes back down it backfires too(only when the engine is warm). I am using the stock Q-jet. I heard recently that this can be caused by the carb being to lean, if so, how can I adjust this? Thanks for the help as always.
Burning Brakes
Joined: Oct 1999
Posts: 1,041
Likes: 1
Re: backfire on deceleration (79corvette)
It can also be an indication of an exhaust leak, so make sure that all of your connections are tight and all the gaskets are appropriately sealed.
You should check your spark plugs and see what they look like. If they are black and fouled with fuel, then you're too rich. If they are white or the metal is somewhat destroyed, you're looking at too lean.
Most carbs have a mixture screw. While I don't work on Q-jets, you can search online for the schematics, and it will show which screws set the mixture vs. idle etc...
You should check your spark plugs and see what they look like. If they are black and fouled with fuel, then you're too rich. If they are white or the metal is somewhat destroyed, you're looking at too lean.
Most carbs have a mixture screw. While I don't work on Q-jets, you can search online for the schematics, and it will show which screws set the mixture vs. idle etc...
Thread Starter
Racer
Joined: Sep 2002
Posts: 293
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From: Englewood CO
Re: backfire on deceleration (flood)
flood
It sputters little bit, then it will pop maybe twice. It is coming from the exhaust.
I did notice that the spark plugs were a little white. I just changed them last night.
It sputters little bit, then it will pop maybe twice. It is coming from the exhaust.
I did notice that the spark plugs were a little white. I just changed them last night.
Pro
Joined: Jun 2002
Posts: 544
Likes: 0
From: La Jolla California
Re: backfire on deceleration (79corvette)
Well this could be normal. When you suddenly decelerate from WOT, for a second your carb will still run really rich. This means some of the gas won't get burnt in the combustion chamber and will pass into the exhaust and finish burning there. If you don't have a restrictive exhaust, it'll make a popping sound when it does this. It happens to me sometimes w/ my true duals also.
However, if this happens everytime you let off the throttle (not WOT), then something else might be going on. Check your timing and carb settings.
[Modified by Langadorf, 4:56 PM 5/6/2003]
However, if this happens everytime you let off the throttle (not WOT), then something else might be going on. Check your timing and carb settings.
[Modified by Langadorf, 4:56 PM 5/6/2003]
Team Owner
Joined: May 2001
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From: Exiled to Richmond, VA - Finally sold my house in Murfreesboro, TN ?? Corner of "Bumf*&k and 'You've got a purdy mouth'."
CI 6-7-8 Veteran
CI-VIII Burnout Champ
St. Jude Donor '06-'10, '13
Re: backfire on deceleration (79corvette)
What did you do to the carb after you put on a better intake, better heads, better exhaust ?
I hope you are giving that motor some more fuel now.....
I hope you are giving that motor some more fuel now.....
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Thread Starter
Racer
Joined: Sep 2002
Posts: 293
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From: Englewood CO
Re: backfire on deceleration (BSeery)
It makes the popping every time I let off the throttle.
I haven't touched changed how much fuel flow the engine is recieving since the upgrades. How can I do this? Do I need to get a better fuel pump, or just change the carb settings(I think the mixture screw is coverd on the stock Q-jet, how can i change that)?
[Modified by 79corvette, 7:32 PM 5/6/2003]
I haven't touched changed how much fuel flow the engine is recieving since the upgrades. How can I do this? Do I need to get a better fuel pump, or just change the carb settings(I think the mixture screw is coverd on the stock Q-jet, how can i change that)?
[Modified by 79corvette, 7:32 PM 5/6/2003]
Team Owner
Joined: May 2001
Posts: 29,745
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From: Exiled to Richmond, VA - Finally sold my house in Murfreesboro, TN ?? Corner of "Bumf*&k and 'You've got a purdy mouth'."
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CI-VIII Burnout Champ
St. Jude Donor '06-'10, '13
Re: backfire on deceleration (79corvette)
It makes the popping every time I let off the throttle.
I haven't touched changed how much fuel flow the engine is recieving since the upgrades. How can I do this? Do I need to get a better fuel pump, or just change the carb settings(I think the mixture screw is coverd on the stock Q-jet, how can i change that)?
What carburetor are you using ?
You would have to change the internal mixture of both the primary and secondary meetering systems for any carb out there. If you don't know carbs, this would be a really good time to get a book and learn, or make some friends with a local person that knows.
Setting a carburetor via instructions over the internet is like trying to instruct someone to paint a painting over a phone.
[Modified by 79corvette, 7:32 PM 5/6/2003]
I haven't touched changed how much fuel flow the engine is recieving since the upgrades. How can I do this? Do I need to get a better fuel pump, or just change the carb settings(I think the mixture screw is coverd on the stock Q-jet, how can i change that)?
What carburetor are you using ?
You would have to change the internal mixture of both the primary and secondary meetering systems for any carb out there. If you don't know carbs, this would be a really good time to get a book and learn, or make some friends with a local person that knows.
Setting a carburetor via instructions over the internet is like trying to instruct someone to paint a painting over a phone.
[Modified by 79corvette, 7:32 PM 5/6/2003]
Melting Slicks
Joined: Jul 2002
Posts: 3,000
Likes: 22
From: Boulder Creek California
Re: backfire on deceleration (BSeery)
I had popping like this and never did much to fix it, but recently I discovered my distributor mechanical advance was corroded stuck. I was getting about 12 degrees initial + vacuum only!! Once this was fixed, and I now get 36 total advance, the popping is completely gone. Might want to give the timing a check.
-Chris
-Chris
Race Director
Joined: Nov 2000
Posts: 11,872
Likes: 12
From: Waterloo ontario Canada
Re: backfire on deceleration (Langadorf)
The idle mixure is not right and the carb is drawing fuel out of the transfer slots causing the backfire. If you see stock cars in the corners shooting flames on deceleration it is fuel being sucked out of the carb. The transfer slots need to be closed up.
[Modified by norvalwilhelm, 8:01 PM 5/6/2003]
[Modified by norvalwilhelm, 8:01 PM 5/6/2003]
Thread Starter
Racer
Joined: Sep 2002
Posts: 293
Likes: 0
From: Englewood CO
Re: backfire on deceleration (norvalwilhelm)
I was driving one night, and revved my engine to scare off the ricers and they told me I had flames coming out that back :reddevil (actually i shouldn't be proud of that), guess I now know why. What are transfer slots and how can I close them up. I have the service manual, are there any good books that someone can recomend for this procedure?
Instructor
Joined: Apr 2003
Posts: 248
Likes: 1
From: Kansas City MO
Re: backfire on deceleration (79corvette)
Might try these:
http://www.amazon.com/exec/obidos/se...447772-6765620
http://www.amazon.com/exec/obidos/se...447772-6765620
Melting Slicks
Joined: Dec 2000
Posts: 3,407
Likes: 2
From: Carmichael CA
Re: backfire on deceleration (79corvette)
I was getting the popping sounds and some backfire out the exhaust until I put in some new exhaust donuts and tightened up the exhaust pipe flange to exhaust manifold bolts...one of them broke and I had to have it removed. It's find now...I get a little rumble when I close down the throttle, but it's under control now...and actually, I kind of like that sound!
Race Director
Joined: Apr 2000
Posts: 17,676
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Re: backfire on deceleration (Surfer69)
I had a similar problem a few years ago, kinda liked the sound however it means you motor is not running properly, I bought a new carb and the backfire went away :D
Thread Starter
Racer
Joined: Sep 2002
Posts: 293
Likes: 0
From: Englewood CO
Re: backfire on deceleration (MotorHead)
I picked up the book that Black Snow recomened. Book seem very informative and now I'm begining to understand a carb. Now I'm gonna try and get the mixture more rich. Just need to read how to do that, metering rods I think.
