Backfiring while accelerating. Help please. - The 1947 - Present Chevrolet & GMC Truck Message Board Network

DevinM · 06-21-2002, 07:04 PM

My 71 SWB has a 350 in her. Last week, she was running fine. But today (no major change in the weather), under acceleration, she wouldn't stop popping. W/T/F. After my drive was done, I let her sit in my garage and idle. I noticed small pops (nothing real noticeable) coming out of the passenger side exhaust pipe. Any suggestions before I have a "deep pocket mechanic" look at it?

tom hand · 06-21-2002, 07:20 PM

First do a "redneck" test. Go out tonight after it is real dark, crank the engine and open the hood. If you have any bad spark plug wires crossfireing you will be able to see it in the dark. Next take a good look at your distrubitor cap for cracks or carbon. To make it pop like that if the ignition is the problem it is crossfireing somewhere. If you can't find a problem there next thing to do is a compression test. I bad valve could cause that and you will know after a compression test. I hope it is something easy.

**COBALT** · 06-21-2002, 07:50 PM

Yeah, sounds like a plug/wire/distributor problem. I don't think it's points or you'd be missing out vs. popping.

70 Jimmy · 06-21-2002, 08:15 PM

The real "redneck" test is to reach in and grab the distributor cap. If it shocks the livin' crap out of you, then you have a bad cap! hehehe

Longhorn Man · 06-21-2002, 09:05 PM

At an idle, put your ear right over the carb, if you hear a popping (not a mis) you may well have a bad intake valve.
Again, at an adle, place a dolar bill over the opening of your tail pipe, if it gets sucked in and blown back out over and over, you probably have a messed up exhaust valve.
Of course you could do a compression check to confirm messed up valves too, but this is quicker.

Model 12 · 06-21-2002, 09:31 PM

Sounds like a sticking valve...

DevinM · 06-21-2002, 10:36 PM

THANKS GUYS! I went out into the pitch black and did the examination. Well, every plug (in the white area of the sparkplug) appeared to have a "faint" glow (rather than a sharp white spark) EXCEPT for one. So I turned the lights on and checked the area. Well, the darn boot was almost hanging off the plug. I re-attached, and away I went....NO MORE BACKFIRING! "I love it when a plan comes together." Thanks again.

tom hand · 06-21-2002, 11:30 PM

Sometimes "redneck" tests work the best. Glad it was a small problem.

chevy72402 · 06-22-2002, 08:05 PM

longhornmail, It is supposed to kinda flutter and sometimes it will go back and forth really fast....

Longhorn Man · 06-22-2002, 09:50 PM

You mean when it is running properly?
On a stock engine, in good tune, it should be wavin like a george washington flag.

06-21-2002, 07:04 PM	#1
DevinM Registered User Join Date: Jan 2002 Location: Deerfield, MA USA Posts: 198	Backfiring while accelerating. Help please. My 71 SWB has a 350 in her. Last week, she was running fine. But today (no major change in the weather), under acceleration, she wouldn't stop popping. W/T/F. After my drive was done, I let her sit in my garage and idle. I noticed small pops (nothing real noticeable) coming out of the passenger side exhaust pipe. Any suggestions before I have a "deep pocket mechanic" look at it? __________________ '71 Chevy C-10 - Current '62 Imapla SS - Traded for my current '73 Nova Hatch - traded for ATC-350X '73 Chevelle Deluxe - First car (R.I.P) DEERFIELD, MA

06-21-2002, 07:20 PM	#2
tom hand CCRider Join Date: Feb 2000 Location: Olive Branch,MS,USA Posts: 2,232	First do a "redneck" test. Go out tonight after it is real dark, crank the engine and open the hood. If you have any bad spark plug wires crossfireing you will be able to see it in the dark. Next take a good look at your distrubitor cap for cracks or carbon. To make it pop like that if the ignition is the problem it is crossfireing somewhere. If you can't find a problem there next thing to do is a compression test. I bad valve could cause that and you will know after a compression test. I hope it is something easy. __________________ 72 GMC Sierra SWB almost finished---- 84 Softail Olive Branch MS

06-21-2002, 07:50 PM	#3
COBALT Senior Member Join Date: Sep 2001 Location: Seattle, WA, USA Posts: 5,817	Yeah, sounds like a plug/wire/distributor problem. I don't think it's points or you'd be missing out vs. popping. __________________ '69 3/4 ton C20 2wd-350ci/TH400 '69 3/4 ton Custom 20 2wd-350ci/4sp Manual '99 2wd 5.7 Chevy Tahoe Seattle, WA.

06-21-2002, 08:15 PM	#4
70 Jimmy aka Crusher, Crushergmc Join Date: Mar 2000 Location: Oregon Posts: 1,834	The real "redneck" test is to reach in and grab the distributor cap. If it shocks the livin' crap out of you, then you have a bad cap! hehehe __________________ "KEEP IT DOWN!" 70 Jimmy 454 2wd 56 GMC Big Window "It's funny till someone gets hurt, then it's freakin' hilarious"

06-21-2002, 09:31 PM	#6
Model 12 Registered User Join Date: May 2002 Location: Knoxville, TN Posts: 20	Sounds like a sticking valve... __________________ If James Dean had a truck, it would be a Cheyenne Super...

06-21-2002, 09:05 PM	#5
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	At an idle, put your ear right over the carb, if you hear a popping (not a mis) you may well have a bad intake valve. Again, at an adle, place a dolar bill over the opening of your tail pipe, if it gets sucked in and blown back out over and over, you probably have a messed up exhaust valve. Of course you could do a compression check to confirm messed up valves too, but this is quicker.

06-21-2002, 10:36 PM	#7
DevinM Registered User Join Date: Jan 2002 Location: Deerfield, MA USA Posts: 198	THANKS GUYS! I went out into the pitch black and did the examination. Well, every plug (in the white area of the sparkplug) appeared to have a "faint" glow (rather than a sharp white spark) EXCEPT for one. So I turned the lights on and checked the area. Well, the darn boot was almost hanging off the plug. I re-attached, and away I went....NO MORE BACKFIRING! "I love it when a plan comes together." Thanks again. __________________ '71 Chevy C-10 - Current '62 Imapla SS - Traded for my current '73 Nova Hatch - traded for ATC-350X '73 Chevelle Deluxe - First car (R.I.P) DEERFIELD, MA

06-21-2002, 11:30 PM	#8
tom hand CCRider Join Date: Feb 2000 Location: Olive Branch,MS,USA Posts: 2,232	Sometimes "redneck" tests work the best. Glad it was a small problem. __________________ 72 GMC Sierra SWB almost finished---- 84 Softail Olive Branch MS

06-22-2002, 08:05 PM	#9
chevy72402 FARMER/RANCHER Join Date: Apr 2001 Location: SD Posts: 597	longhornmail, It is supposed to kinda flutter and sometimes it will go back and forth really fast.... __________________ 1972 chevy super, long box, 402 big block (2,000 miles on rebuild) 2wd tach, tilt, tool box on bedside, a/c, sliding rear window, orange and white. 1972 Chevy Blazer 2wd, tach, tilt, a/c, my first engine overhauling experience, Orange, with white top 2006 Gmc Sierra, XM, Z-71, dark blue, onstar, leather, rear entertainment (audio only) heated seats, (the wife loves that feature) upgraded tires immediately after purchase to Goodyear Wrangler AT/S (hate those rock throwers that come from the factory) BWC

06-22-2002, 09:50 PM	#10
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	You mean when it is running properly? On a stock engine, in good tune, it should be wavin like a george washington flag.



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