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I'm on a roll for receiving good advice here, another air fuel mixture ques

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Ben, Feb 4, 2004.

  1. Ben

    Ben Registered Member

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    I\'m on a roll for receiving good advice here, another air fuel mixture ques

    Another Q-jet question. Been asked before but no response...
    Air/fuel mixture. I have read, "turn out the screws 1 and a half to two turns, and this and that... some say use a vacuum gauge. I used a gauge. Drivers side is turned out about three turns; I adjusted it to the highest reading on the gauge, backed it off to reverify and reset it again. So it's right on target. Now, passenger side. In order to get the highest reading, I had to turn out the screw 7 times before it would come close to matching the drivers side reading. If I turned out the passenger side screw the same amount of times as the drivers side, the reading on the vacuum gauge is very low. Keep turning it out and then it reaches the highest reading at about 7 turns. Normal? Did I adjust it correctly? Suggestions? Carb problem with that many turns?
     
  2. 99firehawk

    99firehawk 1/2 ton status

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    Re: I\'m on a roll for receiving good advice here, another air fuel mixture ques

    You are measuring the vacum at 2 diffrent ports? MY experince with q jets is limited to removing them and throwing them out. BUt it sounds like you are hooking one seid t omanifold vacum and the other to ported.
     
  3. pauly383

    pauly383 Daddy383 Staff Member Moderator

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    Re: I\'m on a roll for receiving good advice here, another air fuel mixture

    I have actually never used a gauge. I put my q-jet on out of the box, set timing proper, then adjusted carb for best idle, then re-time and set idle speed. Hope it helps. Oh and I still pass emissions too /forums/images/graemlins/thumb.gif
     
  4. Goober

    Goober 1/2 ton status

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    Re: I\'m on a roll for receiving good advice here, another air fuel mixture ques

    Without a collection of fancy electronic do-dads in your garage or a customized air/fuel meter in your exhaust, a vacuum guage is the only way to accurately set your idle mixture screws on a Quadrajet. After doing many Quadrajets most folks will figure out how to do it by sound. Although not quite as accurate you can get it pretty darn close.

    When someone tells you "set adjustment screws to 2 -3 turns out" that is just a starting reference point. No 2 needles or seats are cut the same way so every carb will be different. I usually start at 3 turns out on both screws. With the vacuum guage hooked up turn BOTH screws IN 1/2 turn and check vacuum. If the vacuum reading went up then try it again (1/2 turn on BOTH screws). If the vacuum went down, go the other way.

    When you get in the ballpark, fine-tune one side then move to the other ... then go back and check the first side, then the second. Maybe check them both a third time, just to make sure.

    I've seen as much as 1 or 2 turns different on some SERIOUSLY modified Quadrajets but on a stocker, not more than 1 turn. And that would be alot but could be possible if the needle seats are buggered up. Have you modified the idle circuits?


    Where are you reading vacuum from?

    I like to use the brake booster hose that connects to the base of the carb because it is open to both plenums of the intake manifold. I suppose you could also use the PCV connection on the front for the same reason. Whatever you do DON'T connect to vacuum sources that go right into a single intake runner, like the transmission modulator tap. The result will be exactly like you are describing: one adjusting screw will act VERY different from the other. Also, avoid connecting to any of the smaller vacuum sources on the body of the carb unless you are SURE that it provides manifold vacuum.


    If I still had the same problems after adjusting like described above I would doublecheck my ignition system to make sure it was properly setup. Then I would unscrew the adjustment screws all the way and check for crap gumming up the works. Then I would check for any other vacuum leaks.



    Quadrajets are GREAT carbs but they can be a bit of a hassle to setup. /forums/images/graemlins/grin.gif



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