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Backfire on engine braking only - WTF

Discussion in '1969-1972 K5 Blazer | Truck | Suburban' started by homemadesin, Sep 26, 2002.

  1. homemadesin

    homemadesin 1/2 ton status

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    Location:
    Castle Rock, CO
    Specs: 72 CST with '94 SBC 350 crate motor, quadrajet, electronic ignition, stock headers, air pump plumbed and spinning(working -don't know), dual exhaust. The damn thing starts quick - I mean real quick. Runs great as far as I'm concerned, with one exception...it backfires thru the exhaust like a beeattch when I coast, with a massive one after any prolonged WOT stretch. What should I target first? It never backfires during postive acceleration, steady throttle or idle.

    Other info, I haven't made any adjustments to the carb yet (it was a CA car near sea level and I live at 6,200 feet). Also the fan clutch is shot, so I'm driving the fan always.
     
  2. Roostr84

    Roostr84 1/2 ton status

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    I am sure there are guys on here with a lot more experience but my first guess would be to check out simple things like timing etc. I had a car with a pretty pwerful 383 I built and had a similar problem. It was a timing issue for me.
     
  3. jeffro

    jeffro 1/2 ton status

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    Maybe you have an exhaust leak and when you let off the gas the leak sucks fresh air in and the unspent gas ignites.
     
  4. Roostr84

    Roostr84 1/2 ton status

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    I tought of this as well but I would think that the exhaust leak would be noticable upon acceleration as well.
     
  5. 72gmc-burb

    72gmc-burb Registered Member

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    Inspect the air pump check valve for proper operation. If you are able to blow through it both ways, massive back-fires can occur. It looks like a small can usually on the side of the pump attached to same by hose.

    Good Luck!
     
  6. ntaj*ep

    ntaj*ep 1/2 ton status

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    Air pump? Any wow there is an anti backfire valve that should be plumbed in. If its not well.........
    As was alredy said it should look like a little canister on top of the manifold.
     
  7. chulisohombre

    chulisohombre 1/2 ton status

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    smog pump,never seen one of those on a first gen.i dont think they came with one stock.if its on the truck i would get rid of it.anything before 75 doesnt have to be smogged anyway.depending on state of course but overall ive seen they dont.
     
  8. NoSmog73

    NoSmog73 1/2 ton status

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    </font><blockquote><font class="small">In reply to:</font><hr />
    I tought of this as well but I would think that the exhaust leak would be noticable upon acceleration as well.

    [/ QUOTE ]

    We have a winner..... it sounds like an exhaust leak I have the same thing happening and I found out that my connection at the manifold and the exhaust was lose and tightened it up and no more backfiring..
    Just for info the timing being off would lead to "dieseling" and run on after the key is off...mike
     
  9. toms72k5

    toms72k5 Registered Member

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    Franklinton, nc, it\'s a little place 30 miles nor
    I had that problem, after i would be driving for a while, especially over 70, i would let off the the gat and hit the brakes and it would backfire several times, turn out that the smart people at ther shop near my house had tied the spark plug wire to the 2nd cylinder on the drivers side to close to the steering saft and it had worn thru it most of the way, it would fire just fine when i accelerated but as soon as i let off of it, it would backfire thru the exaust, so it might be something that simple.
     

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