What main jets are these??? Holley 600

Fuel and Air Systems

  1. Houseoffubar

    Houseoffubar Well-Known Member

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    image.jpg Hello, I just pulled my Holley 600 vacuum secondary apart and the two main jets are clearly marked “661” not “74” or “68”
    661 on both jets!
    What the hell is this?
    Thanks for any input!
     
  2. abdywgn

    abdywgn dismantler

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    they are a close metering jet(?). A something in between but not quite the same as...oh,I can't explain it. I think they started coming out when the "emissions" carbs came on the scene. I'd look thru the Holley catalog/website/pdf for the precise answer.
     
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    • krazykuda

      krazykuda Well-Known Member FABO Gold Member How-To Section Editor

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      The three numbered jets have flow ratings... The last number is the rate of flow... I'm not sure how many "levels" they have for flow, I believe that it's three...

      That being a 661 means that it flows like a 66 jet at the "1" level of flow...

      You would want to match that one with another 661 so they flow the same...
       
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      • Syleng1

        Syleng1 Karma is real and Life is short... FABO Gold Member

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        Huh? Isn’t “66” the actual flow rate?
        I thought (I get it- krazycuda didn’t make the system up) how can you have different flow rates of the same dia hole? I’m lost. Lol!
         
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        • mbaird

          mbaird mbaird

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          I have some with an H after the number ?
           
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          • 69_340_GTS

            69_340_GTS Well-Known Member

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            Because of the tolerances of the holes. So in the standard sizes, a #66 = approximately 0.066" diameter. Maybe the hole is .0658"-.0662" or something? But they never checked the actual flow. The "close tolerance" jets like your #661 is supposed to flow exactly what a #66 or 0.0660" diameter jet is supposed to flow. They were used to meet emissions regulations.

            EDIT: I may not be 100% correct here. But you get the idea. It really doesn't matter if you use a 66, 661, or 662 in a pre-emisions engine.
             
            Last edited: Sep 1, 2020
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            • Syleng1

              Syleng1 Karma is real and Life is short... FABO Gold Member

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              OOOOOH! now I get it. Holley is only perfect when its Federal regulated emissions.
              If not Federal.. "what ever goes." Think I saw that on their website one day. LOL!:rofl:
               
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              • Mattax

                Mattax Just the facts, ma'am FABO Gold Member

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                Holley Jet numbers are all flow based. You can find two jet sizes with the same diameter bore because the lengths of the bore and the entrance and exit shapes are different.

                A two digit standard jet is allowed 3% variation in flow.
                Three digits ending in 1 or 3 were used by factory calibration, and I too have found H on a jets installed by the factory.
                Three digits were also used for close-limit jets with only 1.5% flow difference allowed.
                Last are three digit metric jets used on model 4360 and 5200 carbs.
                (ref. Mike Urich & Bill Fisher Holley Carburetors and Manifolds HP Books. Holley part No 36-73, 1987 revised)
                 
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                • Houseoffubar

                  Houseoffubar Well-Known Member

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                  Thanks for all the information!
                   
                • MOPAROFFICIAL

                  MOPAROFFICIAL FABO Gold Member FABO Gold Member

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                  That designates high flow.
                  The jet would flow a hair more than rated but not quite up to the next size.

                  I don't know about the 3rd digit, but I'd just assume take the other guys word for it to no real consequence.

                  They've intro'd and discontinued many things over the years, like the anti siphon/anti pull over nozzels.. went from ball to valve pin under the nozzles at the same time...then no brass tubes...then brass tubes...economizer pv's.

                  How old is this carburetor?
                   
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                  • Houseoffubar

                    Houseoffubar Well-Known Member

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                    I’m not sure the age, it is a 9834-1
                    I will research it a bit more to see.
                    I am planning to put it inside a box for a mild blow through turbo setup.
                     
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                    • 69_340_GTS

                      69_340_GTS Well-Known Member

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                      That explains it even better.
                       
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