72 455 GS with factory Qjet. Fast idle 1500rpm .Ideas?

Discussion in 'The Venerable Q-Jet' started by kevin mcculloug, Jul 10, 2010.

  1. kevin mcculloug

    kevin mcculloug 72 GS 455 Convertible

    Fast idle will not change. Turning fast idle screw to either extreme changes nothing.I'm following the chassis service manual, but I'm by myself. so "With engine warm,transmission in drive..." is impossible. Engine is warm, but since the tranny is in park, why would idle be so high? And if it was set wrong, wouldn't turning it in (Clockwise ) to its extreme make things either worse or better? I'm clueless. Thanks for any help.
     
  2. techg8

    techg8 The BS GS

    Fast idle screw (choke cam idle screw) should alter the idle speed with choke cam engaged, in a cold engine / warming up situation.

    Make sure the choke cam is engaging the choke cam follower.

    Does the car come down to a normal idle if you push down on the choke cam, disengaging it? If not, your curb idle / throttle screw could be holding the throttle open further than the fast idle / choke screw.
     
  3. kevin mcculloug

    kevin mcculloug 72 GS 455 Convertible

    No, it does not come down to a normal idle ever.If I turn the curb idle down to anything under 1000 rpms, a miss develops.Curb idle/throttle screw is working fine. Turn it clockwise (in) and the idle increases slowly. Back it out, (counterclockwise) and the idle speed gradually decreases. Idle stop solenoid has been disconnected and reconnected at different times. Its working properly. The only time the idle is under 1,300-1,500 is when its in gear. After a 30 min ride, engine nice and warm, if I put it into Park, rpms rise up to 1,300-1,500.:Do No:
     
  4. kevin mcculloug

    kevin mcculloug 72 GS 455 Convertible

    I've only made adjustments to the engine after it reached operating temp, per chassis service manual.
    I'm waiting for it to cool down to see what the choke cam and follower are doing cold.
    It never gets down to a normal idle out of gear and warmed up. Curb idle screws settings are working fine.
     
  5. bhambulldog

    bhambulldog 1955 76-RoadmasterRiviera

    I had an 73 Olds 455 w Qudrajet. when cranked, cold or warm, it would idle high. To stop the high idle, I would QUICKLY punch the accelerator to the floor (not revving it up, just a QUICK down ,and let off). Then it would idle as it should, until I shut it off. Upon restart, I would have to repeat the procedure. I don't know if this helps, just saying.
     
  6. techg8

    techg8 The BS GS

    How low can you set the idle rpm with the curb idle screw before it stops runnning? I mean can you get a 700rpm idle even if it runs like crap?

    If you can get a low idle (even a crappy one) then I would look at adjusting the idle mix screws to try to smooth it out.

    If you cannot get even a crappy low idle, or if the idle mix screws are unresponsive then I would say there could be a vacuum leak somewhere, or a clogged orifice in the idle fuel circuit.
     
  7. kevin mcculloug

    kevin mcculloug 72 GS 455 Convertible

    I can get a crappy low idle, but I really didn't want to screw with mixture settings. It seems to be running rich anyway, and when I turn the curbidle down into the 750-800 range, I start to get a miss or "pop". If the fast idle cam is operating as it should, taking the screw to both extremes should casue SOMETHING to change, correct? That's not the case. All the way in or all the way out, it doesn't change anything.:Do No:
     
  8. kevin mcculloug

    kevin mcculloug 72 GS 455 Convertible

    I spent 3 hrs last night online reading of other peoples problems . Different makes from Chevelles to Caddys to Oldsmobiles & Pontiacs, to Dodge RVs. Same issues. Previous posts from years ago to days ago on all different forums. Corvette forum had a few posts as well. Problem is, no one posted what fixed what. Once the problem was fixed, they just posted"Got it fixed. Thanks." , and never stated what exactly was the culprit. I have a line on a guy whos local and a Pontiac guy. Supposed to be a whiz with Quads, so I'm going to give him a call Monday. I WILL post whatever I find.
     
  9. Phoenix350

    Phoenix350 Well-Known Member

    That is what I do, I have a fast idle at about 1000-1200rpm. A quick stab and I am down to 500-600rpm. I have a 2gv though so until I get my 4bbl intake on I ain't tuning nothin :bla:

    Hope everything works out for you Kevin!
     
  10. techg8

    techg8 The BS GS

    IF the fast idle cam is operating as it should, the fast idle screw should control RPMs when the cam is engaged with the follower. After all, it is simply opening or closing the throttle via the screw, just like the curb idle screw.

    IF your curb idle screw is set so far in (to attain smooth idle) that the fast idle cam is unable to engage the follower, then the fast idle screw would have no effect.

    Just to test this: I would put the curb idle screw to the crappy 700-800 rpm idle, then manually engage the fast idle cam to its highest step and tinker with the fast idle screw to see if the motor responds.

    --------

    I think you need to get the idle set right without the choke/fast idle involved in any way. Take the choke bracket and cam off if you have to.

    Then once the carb idles as it should, you can add back in the choke and fast idle capability.

    when you say it seems to be running rich anyways, how so? Exhaust smells like Gas? Black smoke out the tailpipes?

    Sometimes a sharp gas smell in the exhaust can be a lean condition.

    Idle mix screws all the way in and then 1.5 turns out is a good starting point for a stock carb.
     
  11. kevin mcculloug

    kevin mcculloug 72 GS 455 Convertible

    I'll give it a try.Thanks.
     
  12. kevin mcculloug

    kevin mcculloug 72 GS 455 Convertible

    Well, all it took was a rebuild by John Osborne. Kinetic Performance rebuilt it and didn't do a very good job. John fixed it up, I bolted it on and its running like a champ.
     
  13. gsjo

    gsjo Platinum Level Contributor

    Take it the shop and sign the pad(do a burnout).:3gears: :3gears:
     
  14. kevin mcculloug

    kevin mcculloug 72 GS 455 Convertible

    1rst dry day, I'M THERE!:beer
     
  15. kevin mcculloug

    kevin mcculloug 72 GS 455 Convertible

    Problem has resurfaced. Doing same thing. When I posted last year, I had driven around a little bit after the carb was rebuilt, but never enough to get it HOT. Warmed up, it runs fine. I set the idle at 700 in gear with the engine warm and in gear. Drive around for 30 mins or so and when I put it in park, the idle jumps to 1200-1500. I turned the idle down to 700 and parked the car as it was late. Next day, start the car cold, and it would barely run at around 450 rpms, so I reset the idle at 700. One idea I heard was that the gas is expanding from the heat and dripping or "overflowing" the fuel bowl when the engine is hot. Guy suggested a 3/8" spacer between the carb and intake. I also remember seeing a 72 that someone put and aluminum heat shield on under their carb. Would either of these help???
     
  16. GSX1

    GSX1 GSX1

    Johns the man He did My Qjet 5 years ago still runs perfect
     
  17. RAbarrett

    RAbarrett Well-Known Member

    I have seen this condition happen several times, especially with a weak throttle return spring. First, make darn sure that the choke is completely off, to a point where the choke coil requires some persuasion to close the choke. Make doubly sure that the fast idle cam is fully retracted, then watch the throttle. The slow idle tang MUST be against the slow idle screw on the driver's side of the carb. I have seen times where the incoming idle air pulled the throttle partially open, especially with a weak throttle return spring. Also note that if the throttle is not fully closed, you will lose partial control of the idle mixture.
    On that note, the idle mixture is fairly easy to adjust, but it must be done in park, and only in park. In gear, the torque convertor will remove some of the engine's response to the settings.
    With the engine FULLY WARM, and as close as possible to the correct idle speed, with the ignition timing and dwell properly adjusted, turn the idle screws out on both sides three turns out.
    Next, using either a tach, or a vacuum gauge, set the screws for either a maximum speed, or a maximum vacuum reading. This is done by making SMALL changes to each screw, one at a time, and noting the engine's response. If the idle speed or the vacuum drops when you make a change, say 1/8 turn in, set the screw back to where it was, and go 1/8 turn out. If the idle rises, do the same thing with the other screw, again noting the response. If the idle rises much above 50 rpm above where it was, reset the idle speed, and repeat the settings with the idle mixture. If you are careful, you will see that the idle speed drops with the slightest "leaning" of the screw. If you see the engine speed stop rising when you turn the screws out, slowly turn the screws in, about 1/16 of a turn, until you just see the idle drop. Your best setting will be where the engine speed has dropped about 5-10 rpm per screw, or a very slight drop in manifold vacuum, of about 1/4 inch of vacuum per screw. Your best idle quality will likely be at about fastest idle to about 1/16 turn lean. Try that, and we'll talk again. Ray
     
  18. kevin mcculloug

    kevin mcculloug 72 GS 455 Convertible

    Thanks Ray. I will give that a shot. I'm also going to install aluminum heat shield and two gaskets. That will aid in getting the carb away from the intake heat a little. Will post results when I do some work. Gaskets are coming from Chicago and somewhere else. NAPA didn't have them in stock.
     
  19. techg8

    techg8 The BS GS

    When the engine experiences that high idle,

    pull the air cleaner off while its running and with your hand, push the secondary air flaps open. The flaps in the air horn, not the throttle blades.

    see if the engine slows down when you do that.

    You can also take a look in there with a flashlight at the top of the secondary throttle blades. See if they are soaking wet with gas.

    What I am getting at is:

    I have seen some Qjets with misaligned or worn out secondary throttle shafts and or blades that dont colse fully and cause a high idle. They expose the secondary bores to manifold vacuum when the air flaps are closed, causing a mess of gasolinge to be pulled from the secondary circuit.

    Manually opening the flaps will lean it back out lowering your idle.

    If this is the case with your carb, the cause could be worn out or misaligned throttle blades on the secondary side, or even a secondary throttle shaft that needs rebushing to fix a worn baseplate.

    Just something to check FWIW
     
  20. kevin mcculloug

    kevin mcculloug 72 GS 455 Convertible

    Still doing same thing. Throttle return spring is fine. Choke coil is NOS and working fine. Rebuilt by John Osborne so no to misaligned or worn out throttle shafts. While running at the high idle and air cleaner off, I can't get the secondary air flaps open with finger pressure. This car also has a factory idle stop solenoid , which is working correctly, adjusted per Service Manual, to achieve the correct idle when WARM. All of the problems occur when the engine is HOT. After a cruise now, engine temp starts climbing while its running at 1500 rpm and not getting any air through the radiator. Temp of the carb with an infared thermometer is in the 160 degree range.
     

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