Where is the carburetor idle adjustment screw on the Rochester ?

Discussion in 'The Venerable Q-Jet' started by green71stage1, Jul 18, 2020.

  1. green71stage1

    green71stage1 Well-Known Member

    Hi Guys,

    I know this is an easy question for everyone but I need to turn down the idle on my 71 Stg 1 455. It has the original Rochester on it. I searched youtube and it was confusing. Is there 2 adjustment screws one regular and one fast idle? I don't want to mes with the wrong screw. Thanks I appreciate the help.
     
  2. rkammer

    rkammer Gold Level Contributor

    Two mixture screws at the bottom front of the carb. Only one idle adjustment screw behind the arm that the throttle cable attaches to on the driver's side. There is a fast idle screw on the passenger side that is only in affect when the choke is partially closed.
     
  3. green71stage1

    green71stage1 Well-Known Member

    Awesome thanks for the help!
     
  4. lemmy-67

    lemmy-67 Platinum Level Contributor

    Keep in mind that those idle mix adjustment screws are feeding a mix of air *and* fuel, not just fuel. At idle, the primary throttle blades should be almost completely closed, and no fuel should be dripping from the primary nozzles. Once your idle speed is set and the engine is warm, tighten the idle mix screws until RPMs drop about 100, then back out ~1/4 turn.

    The fast idle is set by a plastic cam on the passenger side, and has its own separate adjustment.
     
  5. Utah455

    Utah455 Platinum Level Contributor

    Would you happen to have a picture of where the fast idle screw is located? Thanks!
     
  6. hugger

    hugger Well-Known Member

    Only screw on the passside behind the dashpot
     
  7. 1973gs

    1973gs Well-Known Member

    Make sure that the fast idle cam is fully down before adjusting curb idle. It could be sticking or the choke may not be 100% open.
     
  8. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Curb idle adjustment screw,

    DSCF0126.JPG
     
  9. Utah455

    Utah455 Platinum Level Contributor

    Thanks...I know of the idle screw on the driver side where the throttle cable attaches as shown above.

    But is there a second idle screw on the passenger side for the fast idle when choke is engaged (cold engine)? Or is it just the one idle screw on driver side as shown ?

    Thanks!
     
  10. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    There is a fast idle screw on the passenger side, but it is buried in the linkage. You have to look for it.
     
    Utah455 likes this.
  11. Utah455

    Utah455 Platinum Level Contributor

    Think I just found it....Underneath, that's why I didn't see it. Thanks! Something else to check when I get the new thicker gasket installed and see if that solves that heat soak vapor lock issue I was having. I was just adjusting the other idle screw last week which helped at warm up.
    upload_2022-9-1_8-55-41.png
     
  12. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    There is a stepped cam that rotates with the choke. The fast idle screw simply sets the fast idle speed when the choke is engaged. It will have nothing to do with anything else as it ceases to function once the choke opens all the way.

    Adjustment is made with the engine fully warm, the fast idle screw on the lowest step of the cam, and the transmission in DRIVE. Adjust to 620 RPM.

    Fast Idle AdjQJ.jpg
     
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  13. rkammer

    rkammer Gold Level Contributor

    :D with any kind of a medium or lumpy cam, better make that something in the 700-50 range.
     
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  14. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Yup, play with it until you get an actual fast idle on the choke around 1400 RPM.
     
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  15. 1968_GS400

    1968_GS400 Founders Club Member

    Should the fast idle should be 1400 RPM or 620 RPM?

    So you mean fast idle is 620 RPM in drive?

    What should it be in Park?

    Chassis manual says 400 Eng is 600 RPM idle in drive and Fast Idle is plus 20 (so 620 RPM) in drive.
     
  16. 1968_GS400

    1968_GS400 Founders Club Member

    I had a 1968 GS Quadrajet rebuilt by Quadrajet Power. I got it installed and the car started and sounded amazing but was idling way too fast. I don’t have a tachometer but it had to be 2500-3000 RPM. I will try to put a tachometer on tomorrow.

    This was fast idle I guess since it just started. I thought it was way too fast but maybe my perspective is way off because my other carb is so sick/dirty.

    So then I started messing with the fast idle screw. I unscrewed it quite a bit and then the engine died and when restarted doesn’t run but for a few seconds unless I am giving it gas.

    Is there a rule of thumb for how many turns out the fast idle and curb idle screws should be to start? 1 1/2 turns or so?

    I didn’t adjust the idle mixture screws yet and maybe I should have started with those, but the idle was just way too fast to leave it running for long I thought.

    At one point I had it running at a slower initial idle after starting but it was revving the engine by itself up and down a little.

    This is my first time messing with any carburetor settings.

    Am I supposed to set fast idle in drive like Chassis manual says? So should I move fast idle screw back in so it fires up at 2500-3000 RPM, then put it in drive and adjust down to about 620 RPM and then leave it?

    Larry said 1400 RPM above. Is that fast idle RPM in park?

    Thanks in advance for any advice.
     
  17. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    When you first start the engine on the choke, I like to aim for 14-1600 RPM in Park. Touching the gas should drop it down further, and you should be able to put it in gear and drive it. This will require that you play with the fast idle screw to obtain those results. You really can’t use the factory procedures if the engine is modified. Initial timing and vacuum advance will change how fast it idles.
     
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  18. Brett Slater

    Brett Slater Super Moderator Staff Member

    "Adjustment is made with the engine fully warm, the fast idle screw on the lowest step of the cam, and the transmission in DRIVE. Adjust to 620 RPM."

    This excerpt from the chassis manual seems to contradict itself. Either that or I'm totally missing something and I've spent a lot of time both working with my carb & reading up on them the last couple years.

    Once the fast idle cam is back on the lowest (first) step and the choke is fully open, doesn't that take the fast idle screw out of the equation?

    Help me understand.
     
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  19. 1973gs

    1973gs Well-Known Member

    Yes. With the fast idle cam on the lowest step, adjust the curb idle screw on the left side to desired idle. With the choke fully opened, it should be on the lowest step of the fast idle cam, taking it out of the equation for curb idle. Make sure that somebody didn't screw the fast idle screw way in. It should not effect curb idle.
     
  20. Brett Slater

    Brett Slater Super Moderator Staff Member

    The diagram and captions say otherwise. There's zero mention of adjusting the base idle screw in this diagram or directions. That's what's confusing.

    20221211_105319.jpg
     
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