Carburetor Backfire - Team Camaro Tech
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post #1 of 10 (permalink) Old Apr 23rd, 11, 03:33 AM Thread Starter
Matt
 
Join Date: Jan 2011
Location: San Diego
Posts: 6
Carburetor Backfire

Hey Guys. Can any one help me out with this one?

So, today i was driving and in second gear at about 3800 rpm, i started to hear a loud "popping/knocking" noise from the engine. it dissipated as i let off the gas or changed to 3rd. It gradually got worse and started to happen regardless of gear or rpm. (But still much worse and louder at higher RPMs). Seems like I was losing power too. Eventually the engine started sputtering and stalling also. Got it home and took a look under the hood. Found out the loud noise was the carburetor backfiring. I could see the carb puffing out vapor everytime it backfired. It pretty much sounds like I have firecrackers in my engine when i give it gas. So my question is, WHY?!

It seems like a timing issue but the engine sounds good, except for the carb backfiring. I checked the compression and all are consistent (120). Messed with the distributor and makes no difference to the backfiring.

would it be a valve or a valve spring that could be causing this issue? Any thoughts, or insights would be much appreciated.

THANKS!

69 Camaro, 406 SB, TH400, holley double pumper
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post #2 of 10 (permalink) Old Apr 23rd, 11, 06:22 AM
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Ron
 
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Re: Carburetor Backfire

Back Firing through Carb: 1/ Accelerator or Fuel Pump Failing, plain Dirty Carb or Fuel Filter; causing a Lean Out - Hi Possibility. 2/ Mechanical Advance Springs Shot - Hi Possibility. 3/ Loose Distributor and the Timing is backing off - Hi Possibility. 4/ Timing Chain Slipped - Hi Possibility. 5/ Stuck or burnt intake valve - Hi Possibility. 6/ Wiped Exhaust lobe but only when you snap shut the throttle - Possible. 7/ Points need cleaning and adjusted - Possible. 8/ Bad Distributor Bearings - Possible. 9/ Coil Failing or Bad Run circuit in Ignition Switch - Possible. 10/ Vacuum Leak - Possible. 11/ Blown Power Valve - unlikely ... etc.

If you suspect a Valve put a Vacuum Gauge on her.

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Last edited by Z15CAM; Apr 23rd, 11 at 06:46 AM.
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post #3 of 10 (permalink) Old Apr 23rd, 11, 08:12 AM
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Location: Illinois
Posts: 614
Re: Carburetor Backfire

Check with a vacuum gage... http://www.secondchancegarage.com/public/186.cfm
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post #4 of 10 (permalink) Old Apr 23rd, 11, 12:59 PM
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darrell
 
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Location: texas
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Re: Carburetor Backfire

i would assume -its exhaust restriction or cam valve spring broke or exhaust push rod bent -broke rocker--i would pull valve covers before running any more
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post #5 of 10 (permalink) Old Apr 30th, 11, 06:33 PM Thread Starter
Matt
 
Join Date: Jan 2011
Location: San Diego
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Re: Carburetor Backfire

Thanks for the help guys. I took the valve covers off and started it up. I found a rocker that is barely moving. So I am trying to figure out now why the push rod isnt moving (it is not bent). Tightened it down some and got rid of the backfire but the rocker still hardly moves. Do you think it is the lifter? Thanks
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post #6 of 10 (permalink) Old Apr 30th, 11, 06:50 PM
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Terry
 
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Location: SF Bay Area
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Re: Carburetor Backfire

Hopefully it is a bad/collapsed lifter. However don't discount a bad cam lobe. Egads.

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post #7 of 10 (permalink) Old Apr 30th, 11, 07:58 PM Thread Starter
Matt
 
Join Date: Jan 2011
Location: San Diego
Posts: 6
Re: Carburetor Backfire

Ok. took the lifter out (with alot of effort) and found that the bottom of it has been worn down by the cam so that it is now concave. What the hell? So what now? If i replace the lifter, what is stopping it from happening again. Do you think i should pull the cam first? ... i guess i know the answer to that question already, but i am trying to be optimistic
Any advice is appreciated.
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post #8 of 10 (permalink) Old Apr 30th, 11, 08:20 PM
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Join Date: Apr 2008
Location: Illinois
Posts: 614
Re: Carburetor Backfire

Flat cam lobe. Sorry. The motor needs to be pulled and disassembled. All that metal dust can kill the motor. Check the bearings and the piston skirts. I use this to protect my cam...http://www.cam-shield.com/index.html A roller cam conversion fixes the problem the best.
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post #9 of 10 (permalink) Old Apr 30th, 11, 10:38 PM
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darrell
 
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Re: Carburetor Backfire

got to replace the cam and lifters-and use zinc additive--in my opinion you dont have to pull the motor and go thru it --i have done well over hundred without breaking down motor and they have last-but if you dont feel comfortable about it go thru it--i still would chek the valve guide and geometry on that cylinder
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post #10 of 10 (permalink) Old May 4th, 11, 09:27 PM
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Wes
 
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Location: Leander, TX
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Re: Carburetor Backfire

Agree,..wiped lobe. Pull the engine, tear it down, assess damage, dip/clean everything, reassemble with new cam, lifters, bearings, etc.

What cam was this?
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