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Carb is back firing after full throttle, any ideas?
#1435640
05/13/13 10:27 AM
05/13/13 10:27 AM
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Joined: Dec 2006
Posts: 1,178
Indy
FlyFish
OP
super stock
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OP
super stock
Joined: Dec 2006
Posts: 1,178
Indy
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The car runs great, super consistent at the track, but after every pass when I lift off the gas the car back fires. I took the carb apart last week but didn’t find anything wrong. I went ahead and removed my power valve (just to eliminate a variable), plugged it and square jetted it. Raced Saturday….still back fires (but runs great). I found if I take my foot out of it slower, or give it part throttle it usually helps (or stops it completely). Plugs look a bit black as usual, but they always do…for some reason my car gets the best ET/MPH this way. New plugs always start out a nice tan, but get darker as they age (they look like it is too rich, but it runs well so…???).
Carb is a 650dp with a Proform main body (making it a 750), jetted 90 square, on a 410 stroker, pump gas.
Any ideas on what it could be? By the way, this just started at the end of last year, only raced a few times since.
67 Barracuda street car, 408, e85, 1.38 60', 6.44 @105.9 in the 1/8 mile, 10.19 @130.5 in the 1/4...so far....
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Re: Carb is back firing after full throttle, any ideas?
[Re: FlyFish]
#1435641
05/13/13 10:34 AM
05/13/13 10:34 AM
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Joined: Jun 2003
Posts: 52,972
Romeo MI
MR_P_BODY
Master
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Master
Joined: Jun 2003
Posts: 52,972
Romeo MI
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Quote:
The car runs great, super consistent at the track, but after every pass when I lift off the gas the car back fires. I took the carb apart last week but didn’t find anything wrong. I went ahead and removed my power valve (just to eliminate a variable), plugged it and square jetted it. Raced Saturday….still back fires (but runs great). I found if I take my foot out of it slower, or give it part throttle it usually helps (or stops it completely). Plugs look a bit black as usual, but they always do…for some reason my car gets the best ET/MPH this way. New plugs always start out a nice tan, but get darker as they age (they look like it is too rich, but it runs well so…???).
Carb is a 650dp with a Proform main body (making it a 750), jetted 90 square, on a 410 stroker, pump gas.
Any ideas on what it could be? By the way, this just started at the end of last year, only raced a few times since.
Either a spark issue or you have carbon spots(hot
spots) in the chambers that are firing the fuel...
being black I would jet down.. they should look clean...
does it do it with fresh plugs
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Re: Carb is back firing after full throttle, any ideas?
[Re: MR_P_BODY]
#1435642
05/13/13 10:37 AM
05/13/13 10:37 AM
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Joined: May 2004
Posts: 12,387
Taxes & Virus's R-US, NY
Dragula
I Live Here
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I Live Here
Joined: May 2004
Posts: 12,387
Taxes & Virus's R-US, NY
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Pump gas? We have the same issue on our pump gas 10.0 car. It actually blew a hole in a header tube. We are under carburated on ours as it is 512 cubes with a Proform 950 on it...Plugs in ours are really nice. Same thing, if I let out of it gradually, no back fire...But at +135mph, I kinda would like to get on the brakes as soon as I go thru....We have not investigated why ours does it. Curious as to what others think as well. I will be trying out my Prosystems 1000 on it in the next week or two...And we have the tubes repaired.
Last edited by Dragula; 05/13/13 10:43 AM.
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Re: Carb is back firing after full throttle, any ideas?
[Re: MR_P_BODY]
#1435643
05/13/13 10:53 AM
05/13/13 10:53 AM
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Joined: Dec 2006
Posts: 1,178
Indy
FlyFish
OP
super stock
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OP
super stock
Joined: Dec 2006
Posts: 1,178
Indy
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Not sure, I'm going to throw fresh plugs in it for the next race. The odd thing is, the last few times I jetted it down (because I thought the plugs should look cleaner, not black) it slowed down....is it possible that it I jetted it so much past optimum that the MPH picked up???
The proform main body came with 72/84 jets...I'm at 90 square currently. I did back it down a little when I square jetted because I didn't have another set of jets above 90 (it was 82/92 with a power valve for the past 4 years, ran great).
67 Barracuda street car, 408, e85, 1.38 60', 6.44 @105.9 in the 1/8 mile, 10.19 @130.5 in the 1/4...so far....
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Re: Carb is back firing after full throttle, any ideas?
[Re: FlyFish]
#1435644
05/13/13 10:59 AM
05/13/13 10:59 AM
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Joined: Jun 2003
Posts: 52,972
Romeo MI
MR_P_BODY
Master
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Master
Joined: Jun 2003
Posts: 52,972
Romeo MI
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Quote:
Not sure, I'm going to throw fresh plugs in it for the next race. The odd thing is, the last few times I jetted it down (because I thought the plugs should look cleaner, not black) it slowed down....is it possible that it I jetted it so much past optimum that the MPH picked up???
The proform main body came with 72/84 jets...I'm at 90 square currently. I did back it down a little when I square jetted because I was didn't have another set of jets above 90 (it was 82/92 with a power valve for the past 4 years, ran great).
Might need a hotter plug ... shouldnt be black..
so it might want the fuel but it isnt burning all
of it right now(since its black)
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Re: Carb is back firing after full throttle, any ideas?
[Re: FlyFish]
#1435645
05/13/13 11:31 AM
05/13/13 11:31 AM
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Joined: Jun 2003
Posts: 9,225
Charleston
sixpackgut
Drag Week Mod Champion
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Drag Week Mod Champion
Joined: Jun 2003
Posts: 9,225
Charleston
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Quote:
Not sure, I'm going to throw fresh plugs in it for the next race. The odd thing is, the last few times I jetted it down (because I thought the plugs should look cleaner, not black) it slowed down....is it possible that it I jetted it so much past optimum that the MPH picked up???
The proform main body came with 72/84 jets...I'm at 90 square currently. I did back it down a little when I square jetted because I was didn't have another set of jets above 90 (it was 82/92 with a power valve for the past 4 years, ran great).
I have been through the same thing, tried taking everyone's advise here on plugs but what I found works best is AR3935 plug. everyone else seems to run a much colder plug than me but the plugs get black
Gen 3 power 6.22@110, 9.85@135
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Re: Carb is back firing after full throttle, any ideas?
[Re: FlyFish]
#1435647
05/13/13 12:25 PM
05/13/13 12:25 PM
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Joined: Nov 2008
Posts: 660
San Diego
formula S
mopar
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mopar
Joined: Nov 2008
Posts: 660
San Diego
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Quote:
The car runs great, super consistent at the track, but after every pass when I lift off the gas the car back fires. I took the carb apart last week but didn’t find anything wrong. I went ahead and removed my power valve (just to eliminate a variable), plugged it and square jetted it. Raced Saturday….still back fires (but runs great). I found if I take my foot out of it slower, or give it part throttle it usually helps (or stops it completely). Plugs look a bit black as usual, but they always do…for some reason my car gets the best ET/MPH this way. New plugs always start out a nice tan, but get darker as they age (they look like it is too rich, but it runs well so…???).
Carb is a 650dp with a Proform main body (making it a 750), jetted 90 square, on a 410 stroker, pump gas.
Any ideas on what it could be? By the way, this just started at the end of last year, only raced a few times since.
In my opinion that jetting is way to rich we run a hp750 on a 408 with 71 pri jets with a power valve and 78 sec jets no power valve. On the dyno our afr# was 12.7 . No backfire on our car 
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Re: Carb is back firing after full throttle, any ideas?
[Re: 540DUSTER]
#1435650
05/13/13 02:04 PM
05/13/13 02:04 PM
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Joined: Dec 2005
Posts: 1,495
Shelby mi.
JAKE68
pro stock
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pro stock
Joined: Dec 2005
Posts: 1,495
Shelby mi.
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Sounds more like it has an exaust leak or tubes in headers at slip tubes if it has them are leaking.
JAKES AUTOMOTIVE
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Re: Carb is back firing after full throttle, any ideas?
[Re: Cuda_70]
#1435652
05/13/13 02:52 PM
05/13/13 02:52 PM
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Joined: May 2004
Posts: 12,387
Taxes & Virus's R-US, NY
Dragula
I Live Here
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I Live Here
Joined: May 2004
Posts: 12,387
Taxes & Virus's R-US, NY
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Quote:
I agree, maybe a small crack on one of the headers tubes or a bad headers gasket ...
Ours blew a big hole in one of the tubes...
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Re: Carb is back firing after full throttle, any ideas?
[Re: Cuda_70]
#1435653
05/13/13 05:11 PM
05/13/13 05:11 PM
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Joined: Nov 2004
Posts: 19
Sweden
Cuda_70
member
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member
Joined: Nov 2004
Posts: 19
Sweden
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Quote:
I agree, maybe a small crack on one of the headers tubes or a bad headers gasket ...
Sorry I misunderstood, I thought it backfired in the exhaust pipes .... 
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Re: Carb is back firing after full throttle, any ideas?
[Re: Cuda_70]
#1435654
05/13/13 10:20 PM
05/13/13 10:20 PM
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Joined: Jan 2003
Posts: 27,421
Balt. Md
383man
Too Many Posts
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Too Many Posts
Joined: Jan 2003
Posts: 27,421
Balt. Md
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I was going to say a header gasket leak sucking air on decell. That will cause that just like an emission air pump when the dump valve goes bad. Ron
Last edited by 383man; 05/14/13 11:36 PM.
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Re: Carb is back firing after full throttle, any ideas?
[Re: 383man]
#1435655
05/14/13 08:47 AM
05/14/13 08:47 AM
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Joined: Dec 2006
Posts: 1,178
Indy
FlyFish
OP
super stock
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OP
super stock
Joined: Dec 2006
Posts: 1,178
Indy
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Thanks for the info guys, I didn't think about a header leak, I'll check into that next. 
67 Barracuda street car, 408, e85, 1.38 60', 6.44 @105.9 in the 1/8 mile, 10.19 @130.5 in the 1/4...so far....
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Re: Carb is back firing after full throttle, any ideas?
[Re: FlyFish]
#1435656
05/14/13 11:26 AM
05/14/13 11:26 AM
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Joined: Jun 2003
Posts: 52,972
Romeo MI
MR_P_BODY
Master
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Master
Joined: Jun 2003
Posts: 52,972
Romeo MI
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Quote:
Thanks for the info guys, I didn't think about a header leak, I'll check into that next.
Did you SEE it back fire through the carb or are you
hearing something... if your just hearing some popping
then yes I would think header but if you SEE it back
fire through the carb I would think other stuff
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Re: Carb is back firing after full throttle, any ideas?
[Re: MR_P_BODY]
#1435657
05/14/13 11:34 AM
05/14/13 11:34 AM
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Joined: Dec 2006
Posts: 1,178
Indy
FlyFish
OP
super stock
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OP
super stock
Joined: Dec 2006
Posts: 1,178
Indy
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I run exhaust on the car, and it is backfiring through the exhaust.
Edit: Sorry, I just reread this and it was not real clear from the beginning (my bad). The car is backfiring through the exhaust pipes, not up through the carburetor.
Last edited by FlyFish; 05/14/13 11:39 AM.
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Re: Carb is back firing after full throttle, any ideas?
[Re: MR_P_BODY]
#1435659
05/15/13 08:48 AM
05/15/13 08:48 AM
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Joined: Dec 2006
Posts: 1,178
Indy
FlyFish
OP
super stock
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OP
super stock
Joined: Dec 2006
Posts: 1,178
Indy
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Checked the car last night and there is definitely a header leak on the #5 cylinder. I could feel it puff exhaust on my hand on the back side of the header tube near the block...so, I pulled the gasket, but it doesn't look like it was leaking.  I'll replace the gasket this week, then check again...might be cracked on the back side of that tube....if so, that will be a PAIN to reach with the welder without pulling the engine. I'm praying it is just a gasket issue.
67 Barracuda street car, 408, e85, 1.38 60', 6.44 @105.9 in the 1/8 mile, 10.19 @130.5 in the 1/4...so far....
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