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Any Quadrajet experts here?

Discussion in 'Chevy Trucks' started by owen, Jul 11, 2005.

  1. owen

    owen Member
    from florida
    Messages: 34

    Still hasn't snowed in Florida, but I've been hauling lots of Hurricane trash.

    My truck has a Q-jet (#17080212) and auto tranny. Everything runs and works perfect except this one glitch that I can't figure out.

    I happens when I step on the gas enough to drop it into passing gear or perhaps it is when the 4bbl portion opens. After such action the truck is now stuck in a very high idle. I don't have a tach but it feels like about 3000rpm. When I come to a stop it sounds like it's going blow up from running so high. Stomping the pedal does nothing. If I shut the truck off it pops and backfires for a few seconds. When I start it up again it is back to normal.

    The truck has done this since I got it about 2 years ago. I rarely give it enough gas to make it happen. I recently pulled the carb off and did the rebuild kit (a couple of gaskets). Cleaned it thouroughly, reset the idle mix needles to 2.5 turns out and the partial throttle needle to 2 turns out. I found no linkage binding issues and everything looked in top shape.

    It still does it.
  2. Mebes

    Mebes Senior Member
    Messages: 451

    After you shut it off do you just turn the key and start it up without hitting the gas pedal?

    It sounds like the dashpot (I think thats what chev called it)
    It is a little vacuum operated kicker on the side of the Carb that holds the throttle open during deceleration to keep the engine from backfiring after a hard acceleration.
    I'm thinking its either hooked up to the wrong vacuum line or
    If it's thermally controlled the little thermo deal by the thermostat that the vacuum line runs to has failed.
    I may be wrong here but it's just a thought.

    Look for a little round deal with a vacuum line hooked to it on the side of the carburetor that has a piston sticking out of it that pushes (or pulls) on the throttle linkage on the base of the carb and disconnect it and put a screw in the vacuum line and take it for a drive.

    If this doesn't make sense to you than post a picture of the side of the carburetor that the throttle linkage hooks up to and we will see what we can do.

  3. sonjaab

    sonjaab PlowSite.com Addict
    Messages: 1,425

    OWEN.............Do you have one of those "new" electric (computer
    controlled) Q-jets?
    I notice your truck is a 85. My 85 T/A had one.
    They look complicated and $pendy..........Maybe a computer issue?
    geo........... :drinkup:
  4. owen

    owen Member
    from florida
    Messages: 34

    Old style Q-jet here. I think Mebes has it figured out. A vac hose coming off the front of the carb hooks direct to that dashpot on the rear (closest to the firewall).

    Thanks for tip. I'll try it.
  5. justme-

    justme- 2000 Club Member
    Messages: 2,138

    Make ABSOLUTLY sure there is no binding that's is also a big cause for this. A warped casting will also cause it and may be diffacult to notice. Rochester was notorious for crappy castings. We call them "Quadra-bog's" around here.
    Rebuilt the Quadra's on my father's 83 K5 atleast once a year, and on our old 85 K5 every 3 years.
  6. owen

    owen Member
    from florida
    Messages: 34

    Must be mechanical. Just returned from my test drive.

    Disconected ALL vacuum. Both dashpots and even the timing advance. The timing advance actually made the biggest difference. It still got stuck, but at about 1000 rpm less. Could just be due to timing.

    When it was stuck at the lower (2000rpm) mode I was able to "kick" it back down by hitting the pedal so this tells me it's mechanical.

    While it was stuck, no amount of fiddling with flaps on the airhorn assy. made any diff. Pushing and pulling on various linkages also did nothing. I'm guessing it must be getting stuck on one of the butterflies in the throttle body portion.

    Also, the throttle cable linkage is sitting against the idle stop screw while doing this.
  7. Mebes

    Mebes Senior Member
    Messages: 451

    I was thinking about this one last nite in my sleep.

    The Dashpot pulls the throttle linkage shut so disconnecting it would cause the problem all the time.

    When the engine looses vacuum the dashpot moves out. (hard acceleration)
    As the vacuum recovers the dashpot pulls the throttle linkage shut.

    Sorry about the confusion I haven't worked on a quadrajet in a while.

    It still works as I described in my post above, and I still think that it (the dashpot) is your problem.
  8. owen

    owen Member
    from florida
    Messages: 34

    This kinda stuff isn't usually tough for me to solve, but I am stumped on this one. All those dashpots work and seem to be set up correctly according to that "easy to read" paper that came with the carb rebuild kit.

    I'm at the point now of tossing in the towel. What's a good carb if I want economy over performance. Or maybe I should safety wire shut the secondaries and just have "Duo-Jet" :eek:
  9. justme-

    justme- 2000 Club Member
    Messages: 2,138

    That resultant will be not enough power to actually use the truck with anything in it. It's also tough to change a Quadra to anything without changing the intake manifold- there are a couple of spacers/adaptors (at least were) for putting a standard 4 barrell on a quadra manifold, and someone makes a spreadbore quadra clone (no idea if it's any good tho). You also have to think about the computer hookups if this is an emissions carb (probabily is- if it has a blue plug on the top it is) which complicates things emensily.

    Pull a vacuum on the dashpot and check to see if it will hold said vacuum- if it does it's fine (if not there's the problem- it could have a tear and fails to pull properly)

    With the air cleaner off and the engine idling (normally) work the throttle cable from under the hood (cable only not the linkages so it's just like driving conditions) to try to induce the condition watching the linkages for changes. When it's doing it work the linkages a little by hand looking for resistance- look for a linkage that is not where it should be at idle.

    I will almost bet it's mechanical and not the dashpot- if the pot were bad it would not intermitant- the idle would be high all the time or creeping high all the time. It could be the high idle cam.. the choke could be bad (the electric chokes go bad and can partially close the butterfly causing high idle)

    The cable being on the stop means it's not the cable binding- but a linkage could be- I doubt the butterfly itself is unless you fiddles with them when you rebuilt it. Most likely it's a leak internally from a misaligned gasket/torn gasket or warped cover plate/cracked cover plate or a binding linkage.
  10. drifter79

    drifter79 Member
    from pacific
    Messages: 51

    I had a 79 with 350 with the Q-jet went through three different used or rebuilts. I finally broke down and bought the edelbrock Q-jet replacement carb best thing I could have done. Had more power no flat spots even a tiny bit better mileage ran on truck about six years till I sold it. Do not know what they cost now but I know they are not cheap.
  11. festerw

    festerw Senior Member
    Messages: 986

    We replaced the Q-jet on our boat with a Holley ProJection, not cheap but it'll run great and get some decent gas mileage.
  12. owen

    owen Member
    from florida
    Messages: 34

    The Duo-Jet conversion (I wired the secondary so it cannot open) confirmed that the secondary butterfly in the throttle body is the culprit.

    With it wired shut the problem has went away. Could the spring holding that shut be too weak or perhaps not wound tight enough so that just the intake vacuum is holding it open? I cannot find any binding. With the engine not running it test good both hot and cold.

    Wish I understood how these work. The vac/dashpot thing on the right in the drawing above appears to hold the rear snorkle flap closed when vacuum is present. Isn't the vacuum highest at full throttle? Isn't that when you'd want that flap to open up and uncover the secondaries? (just curious) :dizzy:
    Last edited: Jul 17, 2005
  13. justme-

    justme- 2000 Club Member
    Messages: 2,138

    Vacuum is highest at idle. When the plates (butterflies) are closed the engine is sucking air in through them creating vacuum in the intake. When you have the throttle open there is less restriction (high rpm) in the intake thus little vacuum.

    The spring should be fine- if the spring were weak it would always be weak, in other words if the spring is too weak hold the barrels closed when the engines hot, it's too weak to hold it closed when the engine's cold. It sounds like you're loosing vacuum to the secondaries. Check you're vacuum lines for cracks or breaks. If you can get a vac pump (hand held type thing)you can draw down the dashpot and check it to hold a vacuum, also a vac guage on the line that goes to the dash pot will tell you if there's vacuum. (with the car running pull the vac line off the dashpit and put your finger over the end- feel for a vacuum in the line. If not trace it back for the break. I have taken to changing all my vac lines from rubber to urethane since rubber cracks and that can be hard to see, urethane just breaks.
  14. owen

    owen Member
    from florida
    Messages: 34

    That makes a lot of sense. I've felt the vacuum with the engine running on all the hoses, but nothing exact with a guage.

    It sounds like you're loosing vacuum to the secondaries.

    How do the secondaries get the vacuum to open? Is there a tube or hose or something, or are you saying the carb itself is loosing vacuum.
  15. justme-

    justme- 2000 Club Member
    Messages: 2,138

    There are a couple of vacuum lines attached to the carb at several spots- the older the carb the less lines. The line off the dash pot should be short and run from the pot to the carb itself, probabily drawing vacuum from the carb itself (I don't remember). Pull that line directly off the bottom of the dash pot for starters. All vacuum originates off the intake manifold, but I'm not sure of the exact location.

    I checked around for a Qjet book we used to have but no luck.
  16. streetsurfin'

    streetsurfin' Senior Member
    Messages: 770

    I have a 17085408 on a dodge, and here's my thoughts. I didn't read each detail above so forgive me if I missed or repeated something. I'm thinking it may be the choke pull off. It will require the secondaries to open in order to set the choke and this sounds like the reason it doesn't happen when you have them fixed shut. I bet if you shut it down without moving the throttle when it happens again (not wired shut) and check it, you are on the high speed step of the fast idle cam. Most of this is from memory but I think the front vacuumm break (dashpot) is the one that unloads the choke. Depending on how it is choked, electrically and controlled through a resistance timer, as mine is, or some other way, I think it is trying to choke(close the front butterfly) when the rears are opened. Just like when you floor it before startup to choke it. Check that your choke coil is adjusted properly and that it is unloading as it should after warmup. I have the Roe book and papers from rebuild kits so I will look for them and see that I haven't misguided you. Being it is a chevy some of this may not apply, but it might get you closer to a solution.
    Last edited: Jul 20, 2005
  17. SteveB(wi)

    SteveB(wi) Member
    Messages: 70