Quadrajet / 350 Chevrolet
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  1. #1
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    Quadrajet / 350 Chevrolet

    My trusty quadrajet with over 100k since a rebuild won't kick down off high idle when it warms up. I broke a vacuum line, fixed it and when I started it back (cold) the choke closed but now won't open when it gets warmed up.

    It does have electric choke, but I don't know how to test it.
    "You can't out puke a buzzard"

  2. #2
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    Re: Quadrajet / 350 Chevrolet

    Make sure the electric choke is getting 12 volts when the ignition is on. Using an ohmmeter, make sure the coil inside the choke unit has resistance and is not open. Also check the ground for the choke coil.

    On the right side of the carb, behind the choke coil is a cam that has several steps. These steps contact an adjustment screw that determines your high idle speed. With the engine off, hold the throttle about half way open with one hand and manipulate the choke blade with the other, making sure the cam is moving when the choke blade does. this linkage should move freely. The heater inside the choke coil warms a bimetalic spring, which positions this cam. If the heater coil checks good and the linkage is loose, I would next suspect the bimetalic spring is broken and not turning the cam (and opening the choke) as the engine warms up.

    Check the vacuum dashpot, located rearward of the choke coil. Use a hand pump to pull a vacuum on the dashpot, making sure it retracts when vacuum is applied. If it will not maintain vacuum, replace it. Make sure the vacuum hose going to the dashpot is in good shape.

    Hope this helps

  3. #3
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    Re: Quadrajet / 350 Chevrolet

    - have 12 volts to choke
    - coil has resistance
    - good ground
    - cam moves and is free & clean

    I'll have to barrow a vacuum pump to check dashpot, but I did find the vacuum hose going to it is junk. It's flat and cracked over an inch.

    I need to replace all of the vacuum lines so I may as well start there.

    Thanks for the help so far.
    "You can't out puke a buzzard"

  4. #4
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    Re: Quadrajet / 350 Chevrolet

    Well, after going to 3 different parts stores to find the right size vacuum line and answering questions like:

    "does it have power windows?"
    "single or dual headlamps?"
    "short or long wheel base?"

    I finally came home with enough new vacuum line to replace most of my rotten old lines. After a little NAPA now how and my entire allotment of patience for the week used up, I'm happy to report it's back running like a rock again.
    "You can't out puke a buzzard"

  5. #5
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    Re: Quadrajet / 350 Chevrolet

    I use to call them bogguajets, when the secondaries opened you always had that delay before they kicked in..unless you wanted to play around with the jetting...

  6. #6
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    Re: Quadrajet / 350 Chevrolet

    Another issue that sometimes occurs is, the throttle shaft or TS bushings get a wear spot, causing them to hang up.

  7. #7
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    Re: Quadrajet / 350 Chevrolet

    Quote Originally Posted by Hobbytime View Post
    I use to call them bogguajets, when the secondaries opened you always had that delay before they kicked in...
    There is a hint of delay. You want to be darn sure the passing lane is really clear before you jump out there.

    Quote Originally Posted by M J D View Post
    Another issue that sometimes occurs is, the throttle shaft or TS bushings get a wear spot, causing them to hang up.
    This baby has 217k + on it and there's no doubt she's a little loose. But I'm confident $0.44 and 2 minutes of my time has given it new life. I think it was progressively getting worse and explains a lot of issues I had been noticing over the past year or so.
    "You can't out puke a buzzard"

  8. #8
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    Re: Quadrajet / 350 Chevrolet

    Quote Originally Posted by Hobbytime View Post
    I use to call them bogguajets, when the secondaries opened you always had that delay before they kicked in..unless you wanted to play around with the jetting...
    If one knows how to set them up, the Quadrajet can be a very "punch it" responsive carb just by getting the external adjustments right. Bogging is a result of an improperly adjusted secondary air baffle spring.

    Re-jetting, selecting the right primary and secondary metering rods and a couple other well known little tricks can dramatically improve overall performance of one of the best GM carbs ever built IMO.
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  9. #9
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    Re: Quadrajet / 350 Chevrolet

    with an air breather on and tight, spray quick start all around the hoses and base and if the idle changes? You have found your trouble spot. Typically when one line is dry rotted? So are all the rest so replace all. To plug some safety in here I also highly recomend this for brake lines, if one fails replace them all as the rest are not far behind and they fail when your leaning on the brakes hardest. Stay safe fellas

  10. #10
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    Re: Quadrajet / 350 Chevrolet

    Some of them (mostly smog crap) are coming out of a moulded rubber plug with 3-5 hoses. I didn't replace those. A few of them broke off at the moulded plug and I stuck a piece of nylon tubing in the hole to adapt to new hose. I really need those new plug plugs, but the parts stores around here get all when I use words like carburetor, choke pull off, egr valve and sometimes just for giggles - whapolator module.
    "You can't out puke a buzzard"

  11. #11
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    Re: Quadrajet / 350 Chevrolet

    Agree with Duane that you shouldn't be having bogging issues. I haven't touched a Q-jet in probably 20 years other than moving the ones on my shelf around. I still have a bunch of NOS jets and used needles and so forth that I collected over a 5 year period while I was fooling with GM carb'd engines. I absolutely loved Q-jets. Somewhere around is a notebook I kept on settings I had on a range of different GM engines, from a 301 Pontiac to a 454 truck. Set right, those things would really run. The carbs, I mean. I'm not sure there was much you could do to a 301 to get it to run ... although I never got my hands on one of the turbo'd 301's.
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  12. #12
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    Re: Quadrajet / 350 Chevrolet

    Worked with a guy at same dealership that had a '70 Gran Prix, Q-jet equipped 455, TH-400 (with shift kit) and posi. It spanked the butt off a '71(?) 428 Mach 1, 4-speed and posi. Gran Prix was all factory and all he had me do was put the shift kit in and set up the Q-jet. He surprised a LOT of guys with that big silver boat.

    I had a '76 3/4ton long wheelbase Chevy van I ordered/bought new. Q-jet equipped 350 with 327/350hp cam, Headman header/side pipes, TH-350 with shift kit and limited slip rear. Would break the factory Uniroyal radials loose on launch and chirped them on the 1-2 shift.

    I had a '81 1-ton dually with Q-jet equipped 454, TH-400 (again - with shift kit) that would light up and smoke all four radials on bogless to the floor take off. It also broke the 2nd gear clutch pack snap ring lugs out of the housing one fateful day.

    Yup, I too love Q-jets.
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  13. #13
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    Re: Quadrajet / 350 Chevrolet

    Also, nothing like a Q-jet for reasonable fuel mileage with those tiny primaries. Great throttle response from those carbs (properly set up ones, I mean) that puts any fuel-injected engine to shame including the '04 Z06 Vette sitting in my garage. The LS6 in it is a lot faster than the older stuff I once owned, but I do miss the instantaneous throttle response from a carb'd engine and the sound of the secondaries opening up.

    Long live the Spreadbore.
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  14. #14
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    Re: Quadrajet / 350 Chevrolet

    I always heard the term "quadrajunk", not that I'ld know.
    There's something like an Edelbrock on top of my crate these days, that thing doesn't like to return to idle. I believe it has to do with the throttle cable arrangement having a pig'stail corkscrew layout. Sucker gets obnoxious in the colder weather.
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  15. #15
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    Re: Quadrajet / 350 Chevrolet

    Quote Originally Posted by tanglediver View Post
    I always heard the term "quadrajunk", not that I'ld know.
    I've never really worked on quadrajets, american gas guzzlers aren't very common here, but my experience is that most carburettors are good as long as you understand how to and have what's needed to adjust them correctly. Sure, the originally mounted carb designed for good gas mileage on the highway may never be a perfect full race carb, but correctly adjusted it WILL do a good job. Most "bad" carburettors get a reputation to be bad because people don't understand them or sometimes because jets, needles etc. aren't available. Nothing really wrong with the carburettor itself.

  16. #16
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    Re: Quadrajet / 350 Chevrolet

    As for the bog, most likely an accelerator pump issue. Another thing that may help is on the upper air plate for the secondaries is an adjustment. On the passenger side of the carb their is a small Allen screw that sets spring tension on the upper shaft. Increasing that tension should help eliminate the bogging issue.

  17. #17
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    Re: Quadrajet / 350 Chevrolet

    Quote Originally Posted by duaneb55 View Post
    If one knows how to set them up, the Quadrajet can be a very "punch it" responsive carb just by getting the external adjustments right. Bogging is a result of an improperly adjusted secondary air baffle spring.

    Re-jetting, selecting the right primary and secondary metering rods and a couple other well known little tricks can dramatically improve overall performance of one of the best GM carbs ever built IMO.
    agreed, some where I have a book from general motors on how to hot rod the quadrajet carb..after a little tweaking, it was very good at performance..

  18. #18
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    Re: Quadrajet / 350 Chevrolet

    I remember back in the mid 70' S they were on a lot of Firebirds.
    Made a lot of noise and didn't go anywhere.


    ...zap!


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  19. #19
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    Re: Quadrajet / 350 Chevrolet

    Quote Originally Posted by M J D View Post
    As for the bog, most likely an accelerator pump issue. Another thing that may help is on the upper air plate for the secondaries is an adjustment. On the passenger side of the carb their is a small Allen screw that sets spring tension on the upper shaft. Increasing that tension should help eliminate the bogging issue.
    That's one of the basic commonly missed external adjustment but too much tension and the air baffle plate won't open at all. Correct accelerator pump plunger/lever setting is another missed basic.
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  20. #20
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    Re: Quadrajet / 350 Chevrolet

    Quote Originally Posted by duaneb55 View Post
    It spanked the butt off a '71(?) 428 Mach 1, 4-speed and posi.

    Yup, I too love Q-jets.
    I could spank butt all day with my Olds 442 with it's Q-Jet 455 and TH-400. Couldn't really punch it too hard from a stop, trying to move 5K lbs (convertible) from a stop would just result in reducing the diameter of the L-60s. Once rolling it was to the floor then back off a tad at the 1-2 shift to keep the rubber grabbing then back to the floor again. Fun days.
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  21. #21
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    Re: Quadrajet / 350 Chevrolet

    Quote Originally Posted by Hobbytime View Post
    agreed, some where I have a book from general motors on how to hot rod the quadrajet carb..after a little tweaking, it was very good at performance..
    I like Cliff Ruggles book on tweaking Q jets, fairly easy to match the SD455 version using common cores.
    for anything up to 800 cfm much preferred over a Holley fire starter-especially if it is a GM that sees street use..but that is jmo.
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  22. #22
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    Re: Quadrajet / 350 Chevrolet

    Quote Originally Posted by zapster View Post
    I remember back in the mid 70' S they were on a lot of Firebirds.
    Made a lot of noise and didn't go anywhere.


    ...zap!
    My friend had one of those Firebirds. I rode it in it at somewhere around 140 mph ( I think the speedo topped out at 130). It went places... fairly quickly.
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  23. #23
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    Re: Quadrajet / 350 Chevrolet

    Quote Originally Posted by whtbaron View Post
    My friend had one of those Firebirds. I rode it in it at somewhere around 140 mph ( I think the speedo topped out at 130). It went places... fairly quickly.
    Can't help but think of what Ford was offering during 1975 -1976 - The Mustang 2

    While Pontiac was still offering a 455 4 speed T/A- even though it did need a set of 350-400 6X heads a Ram Air 4 cam , set of headers , HEI re-curve, and finally the ( Q-jet tune) for an easy 400 hp and 500 lb torque.

    i guess you (could ) hop up the Ford 2.3 litre 4 or the 2.8 v6 engine- at least they added the 302 in 1975
    Last edited by 455dan; 03-22-2017 at 01:01 AM.
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  24. #24
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    Re: Quadrajet / 350 Chevrolet

    duplicate post
    Last edited by 455dan; 03-22-2017 at 12:41 AM. Reason: duplicate post
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  25. #25
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    Re: Quadrajet / 350 Chevrolet

    Quote Originally Posted by zapster View Post
    I remember back in the mid 70' S they were on a lot of Firebirds.
    Made a lot of noise and didn't go anywhere.


    ...zap!
    There's super stockers that run in the 9's with q~ jet. There 750 and 800 CFm carbs

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