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Thread: Some useful Quadrajet tuning tips

  1. #1
    Vulture of The Western World Eric's Avatar
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    Some useful Quadrajet tuning tips

    This info may be of use to any of you who have a Rochester Quadrajet 4-BBL carburetor and would like to learn more about it - and specifically, how to tune it for optimum performance:

    Rochester's Quadrajet carburettors were a staple of General Motors V-8 powered vehicles from the late 1960s until the switch to electronic fuel injection was finished in the late 1980s. Capable of both fuel economy (for a V-8) and performance they made a name for themselves, although it is sometimes used as a curse.

    The primary barrels of the Q-Jet or are tiny compared to most four-barrel designs, but this is what gives the Quadrajet its gas milage edge. In contrast, the secondary barrels are huge, providing a performance edge. During normal driving the primary barrels are adequate for cruising speeds. The beast comes out when the pedal is depressed further. The secondaries open and there is the slightest amount of delay as the accelerator pump richens the mixture. A Quadrajet equipped car is often distinguishable from other cars by the sound of the engine as the secondaries open. There is a moment of quiet followed by a large increase in exhaust volume, sometimes described as a 'booming' noise.

    Most performance enthusiasts shun the Quadrajet as a stock carburettor laden with useless emissions controls. In reality, the Quadrajet offers performance on a par with most aftermarket carburettors while retaining good driveability and gas mileage. With a little modification most Quadrajets can easily reach 750cfm (cubic feet per minute) airflow. There were many iterations of the Quadrajet, even including some electronic versions produced while General Motors were dragging their feet in changing to electronic fuel injection. The most desirable are the ones produced in the mid-seventies on big-block powered high-performance and luxury cars. These can flow up to 800cfm in the stock configuration.


    AIR VALVE ADJUSTMENT

    A full 90 degreeing opening can inhibit fuel flow from the tubes. By 90 degrees I'm talking about the rear portion of the flap, the front will go past 90 degrees on a correctlyset up carb. The very best q-jets, Ram Air, Super Duty and Ho models were set so that the secondary flaps leading edge is 1.130" to the edge of the opening in the airhorn when fully open. Going beyond this can cause a lean spot as it effects how well fuel is pulled from the tubes and the lower edge of the flap nearly blocks the openings.

    NEEDLE AND SEAT

    I make my own hp needle/seat assemblies. I have found that .130" is the best all around size for hp use. You can go to .149" but it shows no performance gain. It is imperative that the fuel pressure be set at 3.5 to 4 psi, excessive pressure can cause problems with the larger needle/seat assemblies. To make your own hp seat, obtain a numbered drillbit set. Gently install the seat in a soft jawed vise. Drill from bottom to top being carefull to keep the bit straight. No matter how carefull you are a slight "dog ear" will be formed on the seating surface. Use a spare steel checkball and small punch to form a new seat. Vacuum test the seat by installing it in a carb with the float and needle. Invert the carb while holding the float pin in place. Use a vacuum pump hooked up to the fuel inlet via a 3/8 metal line.

    APT is the adjustable part throttle. The carb in the picture should have a metal pin on the front of the power piston. The pin contacts an adjustment screw in the main casting. Most of the later metering rods have a tapered second step. By raising/lowering the adjustment screw you will have fine control of the part throttle mixtures. The later carbs will also have very small idle fuel supply holes in the baseplate. I would enlarge them to .090". Depending on your cam/compression ratio combination you may also need to enlarge the idle tube restrictions, idle channel restriction and add some idle bypass air. I would need to know all engine/drivetrain/vehicle specs to advise further in this area. For secondary rod selection obtain a set of AX .040" rods with long tips. They are a good starting point, I use them in the hot months. I use a custom machined .028" set of rods for cooler weather, and a set of custom .034" rods for moderate weather. As I mentioned earlier the late style carbs make very good hp units, but they will need some help in several areas. Another benefit is that they are 800cfm and work very well on large cid engines.

    GENERAL TIPS

    The secondary choke pull-off is not needed. I would retain the primary pull-off and add an electric choke even if you are eliminating the shaft and flap. This will allow fast idle on cold start-ups and you won't have to "feather" the throttle for 2-3 minutes till it will idle on it's own. In addition to the mods you mentioned I would make sure the carb has a hp needle/seat (at least .130"), brass float and sufficient idle/off idle fuel for your engine. Set the float at exactly 9/32". I would also obtain or modify the primary choke pull-off for a 1.5-2 second release time, most are 3-5 seconds. The secondary airflap will have a limited opening angle, it should be increased by grinding the stop to duplicate the angle of the early HO and Ram Air carbs.



    ADJUSTING AIR VALVE

    Did you see how to adjust your air valve? You might have to take an allen key and a small screwdriver (I use one of those long electrical ones with the red insulation that goes down to the tip which usually came in Stanley screwdriver sets) because you have to tune it until any flat spots disappear. Loosen the spring tension slightly on the air valve until it starts to bog down a little and bring it back up until the bog disappears and a little more and your set. Then it's set for your car and you'll have bog free performance when you floor it from there on and it shouldn't change either. It seems like a lot of work but if you can find a section of isolated road. you'll know pretty quickly if you have a bog. Then you get out and adjust it a little and shut thehood and floor it again.



    ADJUSTING AIR VALVE TENSION

    If you follow that linkage to the arrow on the left in the picture you should see a little flat head screw through the linkage bracket. You'll have to move the linkage back a bit and slide your screwdriver in through the bracket and onto the screw. Directly under the screw is a small allen head grub screw which locks it into position - it's under so you won't be able to see it but trust me it's there. You might have to get a small mirror and a light to locate it for the first time.

    1) use both hands to locate the allen head screw underneath and place the allen key on it but don't turn it!
    2) Hold allen key in place with your left hand while using your right hand to place the flathead screwdriver onto the adjusting screw.
    3) Making sure that you don't let go of the adjusting screw, loosen the allen head grub screw a little bit. Just enough to allow the screwdriver to turn the adjusting screw. The adjusting screw holds tension from a spring so if you loosen the locking screw without holding the adjusting screw it will unwind like a window blind and you'll have to start over again from zero tension (which is another story).
    4) loosen the adjusting screw no more than 1/4 of a turn and preferably 1/8 at a time and tighten the locking screw. You won't be able to see how the air valve butterflies "flop" until you take the screwdriver out and allow the linkage to go free but I suggest that after 1/4 turn you go out and drive it. It should flop open easily but also have enough spring pressure to make sure it closes or it's bog city for you. Floor it under all sorts of conditions and keep loosening it until it starts to bog when you floor it then come back 1/8 of a turn from that point. If you can loosen it 1/4 to 1/2 a turn and still not bog then it needed it. If it bogs on 1/8 to 1/4 of a turn then it was about right and you should leave it. In any case, loosen it until it bogs and 1/8 turn back from that and you should be right. I'll be very surprised if you loosen beyond 3/4 of a turn of where it is now. The leaner the rods the less quickly you can open the airvalves without it bogging. When you have a loose air valve flap you better have the rods to back it up (which we have with the AX/B combo) because there's going to be a whole lot air coming in pretty quickly and it's going to need a lot of fuel.

  2. #2
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    Re: Some useful Quadrajet tuning tips

    Eric,

    Great Q-Jet tips there!

    Have you checked out Cliff Ruggles new Quadrajet book? -

    Here's a link to his book:

    http://www.amazon.com/Rebuild-Modify...e=UTF8&s=books

    The book is fantastic, he does a great job explaining how the Quadrajet works, how to rebuild them, and how to build them up for performance. This book is much better than the very good Rochester Carb book by Doug Roe.

    Cliff is the guy who has built up the qjet's for Jim Hand's winning Pontiac drag cars.

    Here's Cliff's '73 Ventura, which overlaps the '74 GTO discussion:

    http://www.highperformancepontiac.co...c_ventura.html

  3. #3
    Vulture of The Western World Eric's Avatar
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    Re: Some useful Quadrajet tuning tips

    "Have you checked out Cliff Ruggles new Quadrajet book?"

    Not yet - but I will order it today!






  4. #4
    DonTom
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    Re: Some useful Quadrajet tuning tips

    Eric,

    That all sounds like more hassle than junking the carb and putting in MPFI as I did in my RV!

    After we come back from China, I plan on taking the RV out of hibernation. I still have several weeks of vacation time saved up, even after we get back from China!

    -Don-



  5. #5
    Vulture of The Western World Eric's Avatar
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    Re: Some useful Quadrajet tuning tips

    Quote Originally Posted by DonTom
    Eric,

    That all sounds like more hassle than junking the carb and putting in MPFI as I did in my RV!

    After we come back from China, I plan on taking the RV out of hibernation. I still have several weeks of vacation time saved up, even after we get back from China!

    -Don-


    Well, bear in mind that all those steps are rarely necessary; usually it's just a couple of adjustments - and once "set," the Q-Jet will run flawlessly for quite a long time with minimal maintenance.

    FI is fine for the masses - and for emissions control/mileage. It's also great for people who basically don't want to fiddle with their vehicles.

    Carbs, however, are a better "bang for the buck" - and in the hands of someone who knows how to tune them (and enjoys doing so) will produce more power than FI for a lot less coin!

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