Adjustable Power Piston height BELOW retaining collar when fully seated

Discussion in 'The Venerable Q-Jet' started by schwemf, Jul 8, 2019.

  1. schwemf

    schwemf Mike Schweitzer

    I have pulled six Buick Quadrajets apart recently, from years 1969-1971, to check their power piston height when fully seated (i.e. the "down", most lean, position of the primary metering rods within the jets).

    On all of them, the top of the brass cylinder of the power piston sits slightly below the top inner lip of the plastic retaining collar when the piston is fully seated. I expected these to sit slightly (0.020-0.030") above the top inner lip of the plastic retaining collar?

    I don't believe that any of these systems had been adjusted by prior tuners, as all of these carbs still had the staked plug over the APP screw, and all had the "pin" at the bottom of their power pistons.

    IMG_20190708_145440513_HDR.jpg IMG_20190708_145527126.jpg

    Last edited: Jul 8, 2019
  2. HotRodRivi

    HotRodRivi Tomahawks sighted overseas

    They must all have the same length pin sticking out the bottom. I have seen different pin lengths out the bottom . That would change the height. They dont vary much. I think it depends on the metering rods used. longer or shorter will determin pin length at the bottom of piston.
  3. UNDERDOG350

    UNDERDOG350 350 Buick purestock racer

    These are set and seal when the carb was built. It's a rare day that the screw will turn after you get the plug out. My 73 Century screw did turn and cruise speed was very lean on the air/fuel meter. It had a lean pop and lag when rolling into the throttle. I enriched it and runs great now. Most times you will find this screw corroded to the aluminum base plate.
  4. Cliff R

    Cliff R Well-Known Member

    Correct, they can be a bit difficult to remove. Heat and penetrating oil typically required. I remove them here and install an external APT screw with a spring on it so it holds it's adjustment. This is a really nice upgrade for the early carburetors as it allows for custom tuning the part throttle A/F w/o taking the carb apart and changing metering rods.........Cliff

    Attached Files:

  5. UNDERDOG350

    UNDERDOG350 350 Buick purestock racer

    Cliff, sweet fix. This is an important tuning feature that is often ignored.
  6. Cliff R

    Cliff R Well-Known Member

    I find it not only advantageous but pretty much required to use these carbs with this new fuel. The stock calibrations were a tad lean right to start with and the fuel back then was better. It takes about 5-7 percent of this new ethanol enhanced/oxygenated stuff to do the same thing. Having APT is just a good idea and makes things much easier from a tuners standpoint. It's also important to mention that it does not good at all to avoid ethanol for more expensive fuel. I've tested all of it and the non-ethanol stuff is a waste of funds, it's not much if any better than the E-10 stuff. Ethanol really isn't your enemy here, lack of knowledge on tuning your custom engine combo is the real problem.

    To make the problems even worse in many cases at least 95 percent if not more of the folks doing this sort of thing have "modified" their engines to improve performance, or at least that's what they thing they have done. This includes lowering compression for pump gas, aftermarket cams on tighter LSA's, aluminum intakes, cooler thermostats, quicker advance curves, eliminating vacuum advance, blocking off heat crossovers in the heads/intake, etc, etc.

    Those sort of things require a different tune right to start with, then you throw in the fuel quality thing and most folks find their factory carb not up to par for the engine combo.

    I could literally write a book on this sort of thing and the tuning required to make the engine and everything/everyone else involved happy, but being brief here just keep in mind that IF your stock carb isn't making the grade, the problem is probably a combination of variables introduced to the scenario, not specifically the carburetor being inadequate for the application.

    I guess that's why my phone continuously rings off the hook and carbs are always piled high here. These days I prefer to sell custom rebuild/tuning kits, books, and provide technical help vs taking in more carbs than I can every get to.

    Actually, I quit taking in any new for first of July and not going to until everything here is done and gone. At that point going to take a short break, and just take in a few jobs here and there with no turn-around time. So for the most part winding down that part of the business, all my employees have moved on, so I'm the only one doing everything and looking really hard at retirement in the next few years........Cliff
  7. TORQUED455

    TORQUED455 Well-Known Member

    Cliff, congrats on hearing that you’re scaling-back. Hopefully you’ll get to enjoy some free time. You’ve certainly helped a lot of folks out with your Qjet knowledge, experience and parts.
  8. Cliff R

    Cliff R Well-Known Member

    Thanks. Looking forward to slowing down quite a bit, and taking more time off, something I haven't had the luxury of doing for quite some time.......

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