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Backfiring thru carb???

Discussion in 'The Garage' started by Poohbair, Jul 16, 2003.

  1. Poohbair

    Poohbair 1/2 ton status

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    What would make a motor backfire thru the carb? We thought it was timing, but couldnt get it to go away adjusting the timing. We put in a new distributor, cap, rotor, coil, etc...still backfiring. Changed the carb to a different one...still backfiring. Any suggestions?? It is a 383 w/ Vortec heads, Airgap intake, headers, quadrajet, etc...
     
  2. Topdown

    Topdown 1/2 ton status

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    either timing still or you have a couple plug wires switched. - thems my guesses. does the engine run at all?
     
  3. mdm365

    mdm365 1/2 ton status

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    yes it could be wires or timming it can also be valve related like a flat lobe on the cam or anything in the valve train intake side.I found a broken valve spring in my friends truck it poped like crazy into the carb (intake valve spring)I would do a compression check burnt valve? hope this helps. mike
     
  4. Klink K5

    Klink K5 1/2 ton status

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    Like mdn365 said, busted valve spring or flat cam on exhaust side. Easy way to find the hole that the backfire is happening on is to pull one plug wire at a time and floor the carb to see if it backfires. The hole that it dosen't backfire on with the plug wire pulled is the problem. Good Luck /forums/images/graemlins/thumb.gif
     
  5. Poohbair

    Poohbair 1/2 ton status

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    Yes, the motor runs. It seems to run OK down at low speeds, but when you try to get the secondaries open or put a load on the motor (like pulling a hill or something) it starts to backfire and pop thru the carb...
    The motor, heads, cam, carb, distributor, is all NEW! Probably doesnt have 5k miles on it.
    If it is a broken valve spring would we be able to see it if we pull the valve covers? Or possibly feel it by trying to move the springs?
    Is it possible that this could be caused by bad gas? Possibly overheating or anything else (temp guage doesn't work)?
    Just trying to do as much troubleshooting as possible before digging into things like pulling the heads off and stuff.
    Thanks
     
  6. BlazerGuy

    BlazerGuy 3/4 ton status

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    You'd be able to see a broken spring if you pulled the valve covers. I'm pretty sure that bad gas or overheating aren't your problems...
     
  7. Twiz

    Twiz 1/2 ton status

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    [ QUOTE ]
    It seems to run OK down at low speeds, but when you try to get the secondaries open or put a load on the motor, it starts to backfire and pop thru the carb...


    [/ QUOTE ]

    Running lean.
    Backfireing through the carb is a sure sign of a lean mix.
    Sure, cam, spring or anyother odd-ball problems might be present or cause the condition. But with a "new" combination, such as this, the most likely cause is a lean mix.

    Understand that,
    There are 5 independent, allthough interrelated fuel metering systems within the carb.
    These are,
    1 - Cold/Start System. Which include the Choke, Idle system, Transition port and........(not all that interesting)
    2 - Idle System. Which include the Idle adjustment screws, Fuel inlet, Air inlet (both, past the butterflys and past the emulsification inlet ports) and the Transition Ports from Idle Sys. to Main Sys.
    3 - Main Metering System. Which include the Transition Ports, Main Metering Jets, Main Metering Rods or someother fuel en-richment system, Main Emulsification port.
    4- Seccondary Metering System. Which include the Seccondary Transition Ports, Seccondary Metering Jets, Seccondary Mettering rods or someother fuel enrichment systemt, emulsification port.
    5- Exceleration System, Which include the excellerator pump, port, lever, linkage and so-forth.

    Based on the above quote, what is the most-probable cause?
     

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