[TECH] What causes backfire ?????
Thread Starter
Drifting
Joined: Dec 2002
Posts: 1,861
Likes: 2
From: New Zealand
Originally Posted by Cali,68,L-79
Well certainly if your firing order is off it back fires. So if your grounding out you could jump wires and back fire. When does it backfire?
Burning Brakes
Joined: Nov 2004
Posts: 1,028
Likes: 0
From: San Antonio Texas
go to autozone. they sell individual plug wires for about $5.00. replace the one that's cut. if the backfire stops you've found the problem. also, check to be sure all wires are firmly seated to plugs and distributor terminals AND that wires (ESPECIALLY wires adjacent in firing order are not touching). also, check ignition timing. improper timing could easily cause backfire, especially on acceleration.
Burning Brakes
Joined: Aug 2004
Posts: 971
Likes: 7
From: roanoke va
i would check ing. system maybe time for a tune up. if it is backfireing out of the intake it might be a wiped out camshaft lobe i have had that symptom with a wore out cam shaft. also a defective acc. pump in the carb. will back fire thru the carb. just some ideas.
Melting Slicks
Joined: May 2003
Posts: 2,387
Likes: 2
From: Meriden CT
Sometimes backfire is caused by excess un-burned fuel being being pushed into the headers, and igniting in the primary tubes. My carb was too big, and when I let off the accelerator, I would get alot of "popping" in the exhaust. Put a smaller carb on... good to go.
joe
joe
Race Director
Joined: Nov 2000
Posts: 16,416
Likes: 1
From: NC,USA
Backfiring when you let off the gas is a lean condition... the fuel doesnt ignite until its leaving the cylinders... gives you that POP pop pop pop burble when you roll up to a traffic light.
Backfiring through the carb, is bad stuff... blows out powervalves on holleys, lights carbs on fire... ( Nearly lit my hair on fire the other day...) Thats caused by pre-ignition on a cylinder, either a hot spot, a spark plug wire thats arcing to another...
Check your air fuel mixture, do the "sniffer test" (Does it burn your eyes?)
Backfiring through the carb, is bad stuff... blows out powervalves on holleys, lights carbs on fire... ( Nearly lit my hair on fire the other day...) Thats caused by pre-ignition on a cylinder, either a hot spot, a spark plug wire thats arcing to another...
Check your air fuel mixture, do the "sniffer test" (Does it burn your eyes?)
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Race Director
Joined: Sep 2002
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From: San Diego - Deep Within The State of CONFUSION!
Originally Posted by standup
.....it's intermittent.......but I noticed a few times at 2500 to 3000 rpm upon acceleration.........
Running lean is also possible. Check fuel pressure and/or float levels in carb.
Racer
Joined: Jul 2004
Posts: 492
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From: Burlington Ky
Backfiring by definition, whether through the carb or exaust, is always caused by a spark igniting the air/fuel mixture at the wrong time for the air/fuel ratio that is entering the engine, hence is usually a timing problem. It could be a cracked wire, causing that cyl to load up but I doubt it, usually the plug just fouls out. It could be a loose or cracked vacuum line to the advance, a worn dist bushing. A lean condition would cause it, but it wouldn't be intermittant unless something goofy is going on in the carb. Could be as simple as condensation in the dist cap.
Instructor
Joined: Dec 2004
Posts: 200
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Originally Posted by ZD75blue
Backfiring when you let off the gas is a lean condition... the fuel doesnt ignite until its leaving the cylinders... gives you that POP pop pop pop burble when you roll up to a traffic light.
Team Owner
Joined: Jul 1999
Posts: 26,545
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From: Huntersville NC
vette newb - my personal experience was that backfiring thru the exhaust when I let up on the gas... was always caused by an exhaust leak. I had clamped on the entire exhaust myself and there were lots of leaks. Same symptom when I had a broken front pipe. Tightened things up, backfiring stopped. One time I accelerated briskly, came off the gas.... and blew my exhaust tips right off!! 
Tech Contributor
Joined: Aug 1999
Posts: 14,379
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From: At my Bar drinking and wrenching in Lafayette Colorado
Just a minor technical point: Backfire is when the the engine pops up through the carb. Afterfire is when it pops out the exhaust. I think you guys are talking about afterfire - not backfire... Most afterfire is caused by a lean condition. A lean condition can be from multiple sources, but most likely a carb setup issue. Afterfire can also be caused by a cylinder that is not firing at all, thus dumping unburned fuel into the exhaust where it ignites.
Race Director
Joined: Sep 2002
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From: San Diego - Deep Within The State of CONFUSION!
Originally Posted by lars
Just a minor technical point: Backfire is when the the engine pops up through the carb. Afterfire is when it pops out the exhaust. I think you guys are talking about afterfire - not backfire... Most afterfire is caused by a lean condition. A lean condition can be from multiple sources, but most likely a carb setup issue. Afterfire can also be caused by a cylinder that is not firing at all, thus dumping unburned fuel into the exhaust where it ignites.
Thread Starter
Drifting
Joined: Dec 2002
Posts: 1,861
Likes: 2
From: New Zealand
Thanks everyone for their replies
The nearest Autozone to me is 10,474 kms away , but I replaced the plug lead with an older one and separated the leads as best I could but that hasn't worked........thanks, though........
It's through the exhaust......on drivers side, same side as the suspect plug boot...and it's a loud pop.......not a pop-pop-pop.......so "Afterfire" it is........
Just some FYI on my setup:
No smog pump.......
I've got electronic ignition (I didn't think that had vacuum advance????)
New carb in July.......New side exhaust in DEC......ran fine for a week after install......
On SAT I checked the plugs, which are one heat range higher than recommended (don't ask)........all were carbon fouled (rich).......I changed the plugs to recommended range.......warmed up for 5 mins (no afterfiring at idle), drove for 6 minutes, afterfires intermittently under acceleration, but more often in the first three minutes..........same again today after changing the lead........
So I guess it's time to check the timing.......
Originally Posted by greyghost79
go to autozone..........
, but I replaced the plug lead with an older one and separated the leads as best I could but that hasn't worked........thanks, though........
Originally Posted by Jughead
Is it back firing thru the carb or thru the exhaust?
Just some FYI on my setup:
No smog pump.......
I've got electronic ignition (I didn't think that had vacuum advance????)
New carb in July.......New side exhaust in DEC......ran fine for a week after install......
On SAT I checked the plugs, which are one heat range higher than recommended (don't ask)........all were carbon fouled (rich).......I changed the plugs to recommended range.......warmed up for 5 mins (no afterfiring at idle), drove for 6 minutes, afterfires intermittently under acceleration, but more often in the first three minutes..........same again today after changing the lead........
So I guess it's time to check the timing.......
