Quadrajet Question
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500, 472, 425, 368 Discussion, Quadrajet Question in Cadillac Engine Technical Discussion; Hi all, What controls the fast idle on a Quadrajet? I have a 78 Eldorado that runs/starts great in the ...
  1. #1
    awadecki is offline Cadillac Owners Enthusiast
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    Quadrajet Question

    Hi all,

    What controls the fast idle on a Quadrajet? I have a 78 Eldorado that runs/starts great in the cold, but has a minor issue. When the car first starts, the fast idle is on (not sure if there are one or two steps to the fast idle cam on the 78 Eldorados, my 76 has a two-step fast idle). However, if I tap the gas even immediately after the car starts, the fast idle goes off and the car will die. How do I get the fast idle to stay on longer? It stays on for about a minute with my 76 Eldorado which is just about perfect.

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    deVille33 is offline Cadillac Owners Connoisseur
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    Re: Quadrajet Question

    How long have you had the car, or carb? Sounds like a problem with you carb settings. Could be that the coil spring inside your choke control housing isn't set properly. It could be that the vacuum lines aren't properly routed to the thermal control switches. If you have a shop manual or the original under the hood diagram, you can check the routing. Could also be that you have a vacuum leak under the carb base plate. With the engine running spray some carb cleaner around the base of the carb. If the engine speed picks up, you have a leak at the carb base.

  4. #3
    awadecki is offline Cadillac Owners Enthusiast
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    Re: Quadrajet Question

    Hi,

    Thanks for the response. I've only owned the car for about 2-3 months, but it runs perfectly otherwise, and starts fine. Idles smooth as silk so I don't think the base plate is leaking. The car only has 9,000 miles on it. I just want the fast idle to stay on longer. Choke closes fine, it might open a bit slowly if anything. Maybe the primary vacuum break is getting tired?

    What exactly controls the time the fast idle is on?

  5. #4
    carnut is offline Cadillac Owners Master
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    Re: Quadrajet Question

    Behind the choke assembly is a fast idle cam. While holding the throttle wide open,( ENGINE OFF!!) close the choke blade with your other hand. You should see the fast idle cam raise up and with it, a set screw that can be turned in to raise this fast idle speed. I would turn this screw about a quarter turn in to slightly raise the fast idle. I'm thinking that when you blip the throttle to lower the speed, it by passes the other steps on this cam and goes directly to curb idle. I think this an electrically heated choke and if so, it will heat up quickly regardless of engine temp. Older chokes stayed on longer because the heat was from an exhaust manifold. Emmisions laws mandated that the choke come off as soon as possible to lessen rich running. In a Michigan winter, I would let it run on fast idle till the windows were defogged then blip the throttle and drive away. Here in California, once the car starts, we drive it and let the fast idle drop as the car is driven. No need to warm the car up most times. I assume you've sprayed carb cleaner onto all the linkage to make sure nothing is binding as it pivots?

  6. #5
    awadecki is offline Cadillac Owners Enthusiast
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    Re: Quadrajet Question

    Hi, thanks. Yes, I see the fast idle screw. The engine speed when the fast idle is on is just fine, but you're right, it's disengaging too early and letting the engine go down to curb idle. It does have an electric choke heater, but so does my 76 Eldorado. There must be something that controls the amount of time the choke stays on...maybe the choke heater is set too lean and is opening too fast but the vacuum break is not working properly...making it look like the choke is opening slowly...just a theory. Thoughts?

  7. #6
    carnut is offline Cadillac Owners Master
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    Re: Quadrajet Question

    If the choke break diaphram is leaking that would slow down the choke blade getting to vertical, lengthening the time the choke stays on. The choke coil cover, I believe, is riveted. If so, no practical way to adjust short of drilling out 3 rivets. Just turn that screw a quarter turn as described.

  8. #7
    steelybill is offline Cadillac Owners Enthusiast
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    Re: Quadrajet Question

    Carb kits usually have screws to replace the rivets holding the climatic spring cover. You could drill those out if it has them, and use screws, making it adjustable.

  9. #8
    carnut is offline Cadillac Owners Master
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    Re: Quadrajet Question

    The electric choke is adjusted to come off quickly thats why its electric and not dependant on just exhaust heat. If it were mine, I would fine tune the fast idle speed and replace the choke break. Drilling out rivets and moving the cover will just prolong time to disengage which can affect fuel mileage and drivability.

  10. #9
    hardley-etc is offline Cadillac Owners Fanatic
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    Re: Quadrajet Question

    her is a good first paper on Q-jet tuning

    it is a PDF file

    as first it is intiminating but read a few times

    then if you want to learn, print out some pages and try some of the suggestions
    Attached Files

  11. #10
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    albymangled is offline Cadillac Owners Fanatic
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    Re: Quadrajet Question

    thanks for posting the .pdf

    added to my collection.

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