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Backfire through carb

 
racergirl17
I love my Chevy Chevette!
Posts: 3
Joined: 11/08
Posted: 11/18/08 09:45 PM

I am about to loose my mind.  I have a '68 Camaro and recenty put a stock GM crate motor (350 cu / 290 hp) engine into it because I destroyed the original motor.  The car is backfiring through the carb when running in the garage, or under a light/med. load.  The car was backfiring with the old engine as well, so I thought it was the old carb causing the backfire.  I installed a new 650 Holley carb.....but still backfiring.

I brought engine to TDC, and rotor lines up to #1.  I put new wires, cap & rotor on and confirmed wires are installed correctly (1,8,4,3,6,5,7,2), but still no solving this backfire issue.  I have an HEI.  Any ideas or thoughts are much appreciated.

Thank you!   Confused  

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Pontiacman2
Moderator
Posts: 8956
Joined: 09/08
Posted: 11/19/08 05:59 AM

whats the timing set at.  
Pontiacman2
Pontiacman2

Professional Hi-performance engine builder

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racergirl17
I love my Chevy Chevette!
Posts: 3
Joined: 11/08
Posted: 11/19/08 09:06 AM

Timing is set at 8 degrees advanced.  

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SMOKESHOW
I mow my lawn and find Chevys
Posts: 1001
Joined: 09/08
Posted: 11/19/08 02:58 PM

Have you given it more timing?  If so what were the results?  

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racergirl17
I love my Chevy Chevette!
Posts: 3
Joined: 11/08
Posted: 11/19/08 06:21 PM

Timing was at 12 degrees advanced....same result of backfire (with flame) through carb.  

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supersport66
V-6 Camaros rule!
Posts: 49
Joined: 01/08
Posted: 11/19/08 06:57 PM

Sounds like you may need to adjust your valves.Could be one of the intake valves too tight and not fully closing hence the backfire through the carb.
Also make sure the #1 is tdc on the compression stroke when setting the timing.  

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455blockhead
I love my Chevy Chevette!
Posts: 2
Joined: 11/08
Posted: 11/26/08 05:50 AM

I have the exact, exact same problem with my 455. Mine is in a 75 GMC motorhome so it sits for long periods at a time and doesn't get blown out on long trips. I haven't been good about gas treatment so, like you, I thought it might be the carb/fuel. Have you had a chance to look at your valves? I'm starting to think the weak or sticking intake valve theory might be right in my case since my backfiring occurs on the passenger side of the primary barrels. In your case though you have a new engine with these symptoms having occured on both old and new engines. Please let me know if you figure it out. I'll do the same, thanks Confused  

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455blockhead
I love my Chevy Chevette!
Posts: 2
Joined: 11/08
Posted: 11/26/08 12:41 PM

Went back to basics today, compression and vacume tests. For an old '75 455 compression was excellent, 122-130 psi, which meant I didn't have an open or sticking valve. The vacume test fluxed between 10 and 14 Hg so I had a timing problem. The more I thought about it, if the valves were ok and I had backfiring through my carb and under load, then one of the intake valves had to be open when it shouldn't. Had to be timing. Ends up, the diagram I was using in my original GMC manual had #3 and #6 reversed from the actual firing order, a typo...damn! It's fixed and runs great, added fuel stabilizer etc. I would run a compression and vacume test and start there. It will at least answer a lot of basic questions and maybe it will be as simple as my problem since my symptoms and complaint were identical to yours.  

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tractor_andy
I love my Chevy Chevette!
Posts: 3
Joined: 12/08
Posted: 12/03/08 03:12 PM

another thing to do is to check to make sure that the carb is sealed to the intake properly if you can get it to run spray some type of spray can around the carb and see if it is sucked in from below the carb. Be careful as it dangerous but it was the only way to find out that my brothers carb wasn't sealed properly. It was backfiring through the carb constantly and would keep dieing.  

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