Carb Adapter Advice

Discussion in 'Performance & Fuel' started by cisdoz, Oct 30, 2010.

  1. cisdoz

    cisdoz New Member

    I have a Holley on a Weiand dual plane intake. I am going to a Quadrajet which means I need an adapter plate. Should I get a split adapter since the intake is dual plane or will it really make any difference. The engine is a ZZ383 crate engine but I am really after fuel economy.

    1977 GMC 4x4
  2. stephan

    stephan Rockstar 4 Years 5000 Posts

    Hey Cisdoz, Welcome to the GM Truck Club. If it's a true split adapter & seals the 2 halves then yes go with it. The trouble with the open plenum adapters is that they have adverse effects on the manifold mixing & the vacuum pulses end up sucking fuel & air from the wrong venturis.
  3. cisdoz

    cisdoz New Member

    Roger that. That would mean that if there is a seal under the carb, it should be split as well?

    ---------- Post added at 11:11 PM ---------- Previous post was at 09:59 PM ----------

    Thanks for the quick response. After reading your post about the adverse effects, it got me to thinking. It seems everyone agrees using a spread bore carb on a square bore intake is a waste of time and money. I think I will just go with a new setup. I got some work to do.

  4. stephan

    stephan Rockstar 4 Years 5000 Posts

    You meant sealed where you wrote split didn't you??
    If your manifold is a true 180 degree mani. then your split adapter should also be a 180 deg, & the middle runners of both mani. & adapter should be sealed together with a gasket down this center ridge. The same holds true for the top middle runner/ridge of the adapter. It should be sealed to the bottom of the carb with a proper gasket that divides the two haves of the carb left from right. Both gaskets should look like a normal open plenum gasket, but with a line down the center running from front to rear. If all you can find is 4 hole gaskets for a factory quadrajet manifold then just cut the excess out & leave the center strip of gasket materal for both.
    If you happen to have the Weiand dual plane mani. that would also accept the Holley 3 barrel carb, you will need to seal the notch that was machined into the manifold for the secondarys. You can do this with a good quality automotive epoxy, & then sand it down. If you don't seal this, you will suffer from the same maladies when the secondarys are open.

    edit Why are you switching away from Holley?
    Last edited: Oct 30, 2010
  5. cisdoz

    cisdoz New Member

    Based on the advice of several old race junkies, I am switching from the Holley to the Q-jet to improve the fuel economy and basic driveability. I really didn't buy it for a performance street machine. I intend to use it as a daily driver. I purchased a rebuilt Q-jet from Carb Junky's and I am now thinking I should bite the bullet and go with an appropriate intake manifold. I am considering the Weiand 8125 if I can be assured it will mate up with the fast burn heads of the zz383.
  6. stephan

    stephan Rockstar 4 Years 5000 Posts

    LOL I've been there. I run a Holley 650 spreadbore on my truck that I have adjusted for max mileage. I took my Quadrajet off my boat during the 4-5 month off season & put it on the truck hopeing for better mileage. There was no difference, & the driveability was the same.
    A waste of my time & gaskets x 2... ha ha

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