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Failed Emissions!! How do you lean out a Quadrajet.

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Toad, Aug 24, 2000.

  1. Toad

    Toad 1/2 ton status

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    I just failed emissions. Suck! I passed with flying colors last year but I messed up my fuel filter threads and I had to buy a new carb. I bought a Holley Rebuilt Quadrajet (for my 76 350 4bl) that is supposed to be factory tuned. I had to adjust it a little to get it to run right.

    Anyway, I'm running way rich. Suprised me. I have no adjustment for reach/lean on the choke so I have to use the two screws in the front. My question is how far out should these be? Currently they are out 1 1/2 full turns. My old one was out 3 full turns. Some of you say the screws should be out three turns. Is that 3 full turns or 3 half turns equaling 1 1/2 turns.? I used a vacume guage and set it where I got the highest vacume.

    If I understand correctly you tighten the screws to make it run leaner. If the answer to the above questions is 3 full turns I should be running lean with 1 1/2 full turns. What else could it be if my screws are off?

    To paraphrase this. I'm running way rich. What do I have to do to get it to run way leaner.
     
  2. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    OK I just found my manual for my Edelbrock. It has a pretty generic description for setting your idle mixture.

    1)Fully warm up engine and ensure choke is fully open.
    2)Air cleaner in place.
    3)Set desired idle speed with the idle speed screw.
    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 sped by more than 40 RPM then readjust the speed.
    6)Adjust the side OPPOSITE of that in step 4 to get the maximum RPM.
    7)Reset the speed if necesary.
    8)Carefully trim each IMS to again get the maximum idle RPM.
    9)Go leaner just enough to get a 20 RPM drop in sped.
    10)Reset the sped to the desired RPM.
    11)This is a Lean-Best Idle Set. Seting richer than this will not improve idle quality or performance, but could tend to foul plugs.

    I also failed our local emissions test and used this guide to reset my mixture. I failed again on the idle test and took it to have the exhaust gases analyzed by my mechanic. My mixture setting was right on the money, but I had my idle set too low (my 350 is mildly cammed) and this caused my idle numbers to be high. Where I live they have a sign that says they cannot test a vehicle that is idling above 1150 RPM, so my mechanic upped my idle speed to just over 1000 RPM and my truck passed with flying colors! Needless to say as soon as I got home I turned the idle back down to where I like it.
    Sorry for the long response, hope it helps.

    [​IMG]
     
  3. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Oh yeah, turning the screws in leans the mixture and turning them out richens it.

    [​IMG]
     
  4. Toad

    Toad 1/2 ton status

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    Thanks typing that all in for me.

    That reminds me, I usually have my idle a little low. Just a personal preference. Easier to brake, shifts into gear smoother, etc. Maybe that was part of my problem.

    I'll retune my carb tonight or in the morning.
     
  5. Blazer79

    Blazer79 1/2 ton status

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    Also, remember those screws only affect idle mixture. If your test involves revving up, then you'd have to mess with primary jets/rods in order to adjust mixture.

    Good Luck.

    <font color=blue>//////
    http://blazer79.coloradok5.com
    </font color=blue>[​IMG]
     
  6. Toad

    Toad 1/2 ton status

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    I remeber changing my metering rods to the ones in my old carburator. That may be part of my problem. The "Y" that the rods hang on was sticking so I changed it with the old one . The metering rods were smaller on my old carb. I hope I can find the old rods. They are in an old "I can't believe it's not butter" dish. Somewhere. Try saying that with a really bad Flabio impersination. Kind of like a drunk Arnold Swartzenager. It is a lot funnier. Well, maybe not.
     

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