Out of the blue - backfiring through carb while accelerating
#2349190
08/05/17 11:13 PM
08/05/17 11:13 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
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Joined: Nov 2005
Posts: 1,943
San Angelo, Texas, U.S.A.
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I haven't changed or adjusted anything on my engine in two or three years, and it's recently started backfiring through the carb when I accelerate. It's not consistent; it has only happened three times in the last four or five weeks. The carb is a Holley 850 DP and, although I don't remember exactly what I set the timing at, I do remember that it is pretty far advanced (but it's never backfired through the carb like this). The backfires are occurring when the engine is fully warmed and the high temps around here have been around 100 F lately. Any ideas about what might be causing the backfires?
"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]
#2349192
08/05/17 11:16 PM
08/05/17 11:16 PM
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Joined: May 2003
Posts: 96,666 On The Boat, On The Lake, Wa. ...
amxautox
Still Retired. Still Posting on Moparts. A Lot.
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Still Retired. Still Posting on Moparts. A Lot.
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Posts: 96,666
On The Boat, On The Lake, Wa. ...
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Have you been driving it a lot the past year? If not it's been sitting with very little driving?
When my engine was back firing out the carb under acceleration I found deposits in the float bowls from the ethanol gas. Cleaned out the carbs and all was well. Then I had ProMax do their tricks to the carbs, and only use ethanol free gas since.
Tom
"Everyone should believe in something; I believe I'll go fishing."
-Henry David Thoreau
Men and fish are alike. They both get into trouble when they open their mouths
author unknown
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Re: Out of the blue - backfiring through carb while accelerating
[Re: 1968RR]
#2349204
08/05/17 11:24 PM
08/05/17 11:24 PM
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Joined: Nov 2005
Posts: 1,943 San Angelo, Texas, U.S.A.
1968RR
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I drive it a couple of times a week, year-round. I may take the carb apart soon to check it out. Could a bad accelerator pump cause this? The car's been running normally otherwise.
"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]
#2349210
08/05/17 11:30 PM
08/05/17 11:30 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
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up yours
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If it ends up you need ethanol free, Schneider gas on Abe and Harris has it in 91 octane only.
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: Supercuda]
#2349212
08/05/17 11:34 PM
08/05/17 11:34 PM
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Joined: Nov 2005
Posts: 1,943 San Angelo, Texas, U.S.A.
1968RR
OP
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OP
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If it ends up you need ethanol free, Schneider gas on Abe and Harris has it in 91 octane only. I used to fill up at Schneider's all the time back when they were the only place in town with 92 octane. Once they switched to 91 octane, I just started filling up wherever. I never knew their gas was ethanol-free. I guess I'll have to start filling up there again. Thanks for the tip. ![up up](/ubbthreads/images/graemlins/custom/thumbs_up.gif)
"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]
#2349220
08/05/17 11:50 PM
08/05/17 11:50 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
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Posts: 14,889
up yours
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just the 91 octane is what I understand
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: screamindriver]
#2349229
08/06/17 12:11 AM
08/06/17 12:11 AM
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Joined: Nov 2005
Posts: 1,943 San Angelo, Texas, U.S.A.
1968RR
OP
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Holley...Make sure the accelerator pump diaphragms didn't dryrot/expand/rupture etc...I need to run the green ones since ethanol was forced on us... I'll go through the carb in the next day or so and check out the pump diaphragms. I'll also check the timing to make sure that it hasn't somehow found a way to advance. Any other ideas?
"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: Dave Hall]
#2349246
08/06/17 12:35 AM
08/06/17 12:35 AM
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Joined: Nov 2005
Posts: 1,943 San Angelo, Texas, U.S.A.
1968RR
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As said before. Check the plugs and make sure the gaps aren't too wide. Plugs look good. They only have about 1500 miles on them.
"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]
#2349249
08/06/17 12:39 AM
08/06/17 12:39 AM
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Joined: Nov 2003
Posts: 36,041 Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
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Lincoln Nebraska
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Could a bad accelerator pump cause this? yes check the A/P tip in. should be a nice/steady immediate stream when you jerk the throttle. you might try it many times to get it to (hopefully) recreate & speaking of this, is the backfiring at WOT or part throttle?
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: Kern Dog]
#2349250
08/06/17 12:40 AM
08/06/17 12:40 AM
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Joined: Nov 2005
Posts: 1,943 San Angelo, Texas, U.S.A.
1968RR
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OP
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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. The idea that the cam could be going flat crossed my mind, but I haven't seen any glitter in the oil or the oil filter (I always cut the filters open), although I haven't changed the oil recently. I'll check the carb first, and if it checks out I'll do a compression check. The cam has over 2000 miles on it and I use oil with plenty of ZDDP, but you never know...
"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: RapidRobert]
#2349251
08/06/17 12:41 AM
08/06/17 12:41 AM
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Joined: Nov 2005
Posts: 1,943 San Angelo, Texas, U.S.A.
1968RR
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OP
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Could a bad accelerator pump cause this? yes check the A/P tip in. should be a nice/steady immediate stream when you jerk the throttle. you might try it many times to get it to (hopefully) recreate & speaking of this, is the backfiring at WOT or part throttle? It is backfiring at part throttle.
"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]
#2349257
08/06/17 12:47 AM
08/06/17 12:47 AM
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Joined: Nov 2003
Posts: 36,041 Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
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Lincoln Nebraska
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would you have another carb you could swap in for some testing?
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]
#2349269
08/06/17 01:16 AM
08/06/17 01:16 AM
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Joined: Nov 2005
Posts: 1,943 San Angelo, Texas, U.S.A.
1968RR
OP
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OP
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would you have another carb you could swap in for some testing? Unfortunately, no. The engine's still pretty warm, so I'm going to wait until tomorrow to remove and go through the carb. Other than the pump diaphragms and junk in the bowls, is there anything else that I should look for?
"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]
#2349271
08/06/17 01:18 AM
08/06/17 01:18 AM
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Joined: Jan 2003
Posts: 2,744 541 slobovia
A990
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master
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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. The idea that the cam could be going flat crossed my mind, but I haven't seen any glitter in the oil or the oil filter (I always cut the filters open), although I haven't changed the oil recently. I'll check the carb first, and if it checks out I'll do a compression check. The cam has over 2000 miles on it and I use oil with plenty of ZDDP, but you never know... Do the filter inspection first, that way if its good you wont toss 4 qts of perfectly good oil. I don't want to know how much good oil I've dumped over the decades.
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Re: Out of the blue - backfiring through carb while accelerating
[Re: 1968RR]
#2349273
08/06/17 01:21 AM
08/06/17 01:21 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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Not to my knowledge & I'm thinking that it highly likely it might be a part time blockage (debris) in the AP circuits' passages. what I would do before pulling the carb is open it up & blast out the carb circuits with brake kleen. Holler how it goes.
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]
#2349280
08/06/17 01:35 AM
08/06/17 01:35 AM
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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
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Posts: 1,943
San Angelo, Texas, U.S.A.
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Not to my knowledge & I'm thinking that it highly likely it might be a part time blockage (debris) in the AP circuits' passages. what I would do before pulling the carb is open it up & blast out the carb circuits with brake kleen. Holler how it goes. I'll check it out and get back. ...genuinely appreciate the help.
"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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