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Q-Jet mixture screws

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Burbinator, Sep 17, 2001.

  1. Burbinator

    Burbinator 1/2 ton status

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    This might sound like a stupid question, but is turning the screws "in" leaning or richening the mixture? It's been a while since I tweaked around on a Q-Jet and I can't remember if the screws control the amount of air or the amount of fuel. Thanks!

    - Alan

    '82 K10 Suburban Silverado (AKA U.S.S. Nimitz)
     
  2. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    I'm not 100% sure but I know that all the screws do is change the mixture on the idle circuit. As soon as you give it some gas they have no more affect on mixture. Part throttle is controlled by your metering rods and jets.

    Rene

    <font color=green>Dyslexics of the world...UNTIE!</font color=green>
     
  3. mike reeh

    mike reeh 1/2 ton status

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    turning them IN or CLOCKWISE leans out the mixture.. they control the fuel flow :)
     
  4. DMK

    DMK 1/2 ton status

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    In lean, out rich.
     
  5. Burbinator

    Burbinator 1/2 ton status

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    True. The idle circuit is one of my concerns due to the exhuast smelling "fuelie", for lack of a better word, when at idle, since it idles a little lopey for a stock profile cam. Of course, that could be connected to my pinging problem in my other post.

    - Alan

    '82 K10 Suburban Silverado (AKA U.S.S. Nimitz)
     
  6. Burbinator

    Burbinator 1/2 ton status

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    In lean, out rich...made a mental note and wrote it down this time. That's what I thought, but wanted to be 100%. Any thoughts on the average number of turns out in a stock application? Thanks for the info, same to mike reeh and trustyk5...

    - Alan

    '82 K10 Suburban Silverado (AKA U.S.S. Nimitz)
     
  7. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    I've heard 1 1/2 turns out is close...but my quad wouldn't run with less than 3 turns. *shrug* It prolly needed a rebuild though.
    If you haven't already got it you should find a book written by Doug roe, called Rochester Carburetors. Loaded with all sorts of good info. Also if you bought the 1906 Edelbrock Quad new it should have come with a manual that outlines the proper way to set your mixture.

    Rene

    <font color=green>Dyslexics of the world...UNTIE!</font color=green>
     
  8. DMK

    DMK 1/2 ton status

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    Yeah 1 1/2 is a good starting point.
     
  9. Burbinator

    Burbinator 1/2 ton status

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    I've heard such great things about that particular book that I ordered it on Friday. My screws are currently out 3 turns (hence the "fuelie" smell), in an effort to lose that momentary off idle bog it had at 1.5 turns and that friggin' pinging (unrelated to the pinging, I know now). I've since misplaced or my wife threw away my Edelbrock manual since adding that carb. Guess I'll wait for that book and keep tweaking on it. Thanks for the input.

    - Alan

    '82 K10 Suburban Silverado (AKA U.S.S. Nimitz)
     
  10. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Here's the steps outlined for my 1406 edelbrock. Seting mixture should be the same.

    1. Fully warm and ensure choke is fully open.

    2. Air cleaner in place.

    3. Set desried idle speed.

    4. Adjust the IMS on one side to get the maximum possible rpm. Do not go rich beyond the maximum speed point.

    5. If the above changed the idle speed more than 40 rpm, the readjust the speed.

    6. Adjust the opposite side of that in step 4 to get the maximum rpm

    7. Reset the speed.

    8. Carefully trim each IMS to again get the maximum idle rpm.

    9. Go leaner just enough to get a 20 rpm drop in idle speed

    10. Reset the idle speed to the desired speed.

    11. This is the Lean-Best idle setting. Setting richer than this will not improve idle quality or performance, but could tend to foul plugs.

    Using this guide I got my Jimmy through emissions testing with no real problems. I did fail once but my idle speed was too slow and the cam's overlap was the culprit. I raised the idle speed to 700 rpm and passed. Before my re-test I had it run on an 4 gas analyzer and found out my Air fuel ratio was 14.8 to 1. Thats damn near perfect using Edelbrocks instructions![​IMG]

    Hope this helps!

    Rene

    <font color=green>Dyslexics of the world...UNTIE!</font color=green>
     
  11. Burbinator

    Burbinator 1/2 ton status

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    Thanks for all your help, Rene, and all of the others that have helped me in my posts. I'm sure that I'll get this thing running right and passing emissions when I have some time to tweak my carb and try that ATF trick in the next couple of days. I'll let you know the results!

    - Alan

    '82 K10 Suburban Silverado (AKA U.S.S. Nimitz)
     

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