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Backfire After Q-Jet Rebuild

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by johnny reb, Jun 15, 2000.

  1. johnny reb

    johnny reb 1/2 ton status

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    Recently rebuilt my carb (Q-Jet) and now I have a backfire through the exhaust. I changed plugs, wires, dist. cap, and rotor and that helped a bit, but it's still there. Also, since I rebuilt the carb I have a hollow exhaust note and loss of power just before the truck shifts into second gear. Could it be that the mixture screws aren't adjusted properly? If so, how should I adjust these?
     
  2. Burt4x4

    Burt4x4 3/4 ton status

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    I had a similar backfire and the ultimite fix was to advance the enitial timming to 10 I was at 4 and my total timming is around 34 at 3000rpm.
    For you mikture screws GRIM is the MAN on that but it goes something like this get a vac gage and turn one scew in until the idel is ruff then bring it out until you achive the highest vac then to the same to the other side then readjust the idel speed.
    Good Luck

    Rock ON![​IMG] 72 K5 [​IMG] Led Zeppelin
    http://burt4x4.coloradok5.com
     
  3. Blazer79

    Blazer79 1/2 ton status

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    That sounds like timing problem to me. Check it with a timelight before anything.

    The description Burt gave you is pretty accurate, but it refers to setting the idle mixture screws. The other mixture screws affect off-idle and higher rpms. Those are usually sealed and I don't recommend messing with them.

    You might want to check the power piston. Maybe it's getting stuck. Remove the top of the carb and check that the piston is free to move. Also be careful when replacing the gasket so it doesn't bind it.

    When my Edelbrock Q-jet was new, I had a similar problem. I found out that it had a small piece of some kind of dirt or rubber BEHIND BOTH the primary jets. Of course I was lacking power.

    Good luck

    <font color=blue>//////
    http://blazer79.coloradok5.com
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