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Re: Out of the blue - backfiring through carb while accelerating
[Re: 1968RR]
#2349282
08/06/17 01:42 AM
08/06/17 01:42 AM
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Joined: Nov 2003
Posts: 36,041
Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,041
Lincoln Nebraska
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that's what we are here for & everything I know (& am still acquiring) has come from 13 yrs here on Moparts (as an ex employee of Speedway motors I did not learn a single thing there!)
live every 24 hour block of time like it's your last day on earth
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Re: Out of the blue - backfiring through carb while accelerating
[Re: RapidRobert]
#2349308
08/06/17 03:16 AM
08/06/17 03:16 AM
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Joined: Aug 2003
Posts: 1,145
ontario calif
ns1aar
super stock
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super stock
Joined: Aug 2003
Posts: 1,145
ontario calif
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I've had plug wires do this
NS1AAR
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Re: Out of the blue - backfiring through carb while accelerating
[Re: Andrewh]
#2349529
08/06/17 03:46 PM
08/06/17 03:46 PM
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Joined: Jan 2003
Posts: 15,565
Omaha Ne
TJP
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 15,565
Omaha Ne
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A lot more info would be helpful, Accelerating ? From a dead stop, cruise, part throttle, WOT etc. What RPM range ?? 
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Re: Out of the blue - backfiring through carb while accelerating
[Re: TJP]
#2349552
08/06/17 04:38 PM
08/06/17 04:38 PM
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Joined: Nov 2005
Posts: 1,943
San Angelo, Texas, U.S.A.
1968RR
OP
top fuel
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OP
top fuel
Joined: Nov 2005
Posts: 1,943
San Angelo, Texas, U.S.A.
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I just pulled the carb and the pump diaphragms and bowls both look good. When reassembling the carb, I noticed that the rubber vacuum cap on the small base plate port under the front bowl was cracked and likely was causing a small vacuum leak.
Could this have been the culprit?
"When I'm in a slump, I comfort myself by saying if I believe in dinosaurs, then somewhere, they must be believing in me. And if they believe in me, then I can believe in me." - Mookie Wilson
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Re: Out of the blue - backfiring through carb while accelerating
[Re: TJP]
#2349555
08/06/17 04:39 PM
08/06/17 04:39 PM
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Joined: Nov 2005
Posts: 1,943
San Angelo, Texas, U.S.A.
1968RR
OP
top fuel
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OP
top fuel
Joined: Nov 2005
Posts: 1,943
San Angelo, Texas, U.S.A.
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A lot more info would be helpful, Accelerating ? From a dead stop, cruise, part throttle, WOT etc. What RPM range ?? Part throttle from a dead stop. It hasn't been consistent enough for me to check the RPMs, but I'd estimate between 2500 and 3500.
"When I'm in a slump, I comfort myself by saying if I believe in dinosaurs, then somewhere, they must be believing in me. And if they believe in me, then I can believe in me." - Mookie Wilson
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Re: Out of the blue - backfiring through carb while accelerating
[Re: 1968RR]
#2349563
08/06/17 04:45 PM
08/06/17 04:45 PM
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Joined: Sep 2007
Posts: 14,889
up yours
Supercuda
About to go away
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About to go away
Joined: Sep 2007
Posts: 14,889
up yours
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I just pulled the carb and the pump diaphragms and bowls both look good. When reassembling the carb, I noticed that the rubber vacuum cap on the small base plate port under the front bowl was cracked and likely was causing a small vacuum leak.
Could this have been the culprit?
Quite possibly, especially if you were already borderline lean, either due to jetting or ethanol content.
They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.
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Re: Out of the blue - backfiring through carb while accelerating
[Re: 1968RR]
#2349587
08/06/17 05:41 PM
08/06/17 05:41 PM
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Joined: Feb 2005
Posts: 28,312
Cincinnati, Ohio
Challenger 1
Too Many Posts
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Too Many Posts
Joined: Feb 2005
Posts: 28,312
Cincinnati, Ohio
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I just pulled the carb and the pump diaphragms and bowls both look good. When reassembling the carb, I noticed that the rubber vacuum cap on the small base plate port under the front bowl was cracked and likely was causing a small vacuum leak.
Could this have been the culprit?
Possible, but when I get backfire out of the carb under load it means my spark plugs are dirty. Some times I bead blast mine clean, once. Then I need new ones. Vacuum leaks cause backfire on decel most times. My plugs used to last for years until I hooked my manual choke for warm up. If I am not careful I will foul out a plug or two if I use the choke too long and get a little back fire out of the carb under load. Fresh plugs it is running clean again. 1500 miles? plenty of time to partially foul a plug ...IMO. If you did not find any other reason. Did you actually pull those 1500 mile plugs to be sure?
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Re: Out of the blue - backfiring through carb while accelerating
[Re: 1968RR]
#2349628
08/06/17 06:59 PM
08/06/17 06:59 PM
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Joined: Nov 2005
Posts: 1,943
San Angelo, Texas, U.S.A.
1968RR
OP
top fuel
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OP
top fuel
Joined: Nov 2005
Posts: 1,943
San Angelo, Texas, U.S.A.
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I replaced the cracked port cap and took the car out for a short drive, and there weren't any backfires. Of course, that doesn't mean that the issues is resolved, but it is a little encouraging.
I pulled the #2 and #6 plugs (as sort of a spot check) about a month ago, and both of them looked good. But if the backfiring persists, I'll go ahead and replace the plugs since I have a new(ish) set lying around.
"When I'm in a slump, I comfort myself by saying if I believe in dinosaurs, then somewhere, they must be believing in me. And if they believe in me, then I can believe in me." - Mookie Wilson
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Re: Out of the blue - backfiring through carb while accelerating
[Re: Kern Dog]
#2351080
08/09/17 12:43 AM
08/09/17 12:43 AM
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Joined: Jan 2003
Posts: 5,791
Hamilton, Ontario Canada
Magnum
master
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master
Joined: Jan 2003
Posts: 5,791
Hamilton, Ontario Canada
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I would do a compression check to see if there is any excessive variance between cylinders. When a camshaft lobe starts to fail, it is often an exhaust lobe. When THIS happens, the exhaust looks for any exit it can find. Sometimes under heavy throttle, the exhaust gets pushed up through the intake valve and into the intake manifold. I've had a pushrod through the exhaust rocker arm do the same thing.
69 Super Bee, 93 Mustang LX, 04 Allure Super
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