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Thread: holley carb vaccum to mechanical

  1. #1
    Senior Member EXPERT BUILDER
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    holley carb vaccum to mechanical

    can i convert vacuum secondaries to mechanical? on a budget my vaccum secondaries wont open on either of my carbs 750 and the 600 i can open by hand but it wont open at w.o.t
    any ideas
    thanks logan

  2. #2
    Senior Member DYNO OPERATOR
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    Holley

    Your vacuum secondaries are on demand so to speak. To quicken the response to demand you can get a Holley spring assortment and change the spring under your secondary diaphram. You will never see the vacuum secondary open all the way in the shop just wacking the throttle as there is not enough demand on the carb to open all the way.

    To convert to machanical secondary you would also need a secondary accelerator pump to go along with that. It would be better to sell what you have and purchase a mechanical secondary carb. JMO

  3. #3
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    the 600 wont open at all and the 750 opens a little we were reving this thing to close to 7 grand

  4. #4
    Senior Member RACING JUNKIE BEAST477's Avatar
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    Quote Originally Posted by mrbigtrucks
    the 600 wont open at all and the 750 opens a little we were reving this thing to close to 7 grand

    Wow,is this on your mild 350 :?:
    BEAST477






    Only a rich man can afford to use cheap parts.

    http://www.racingjunk.com/profile/177549

  5. #5
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    yeah the 750 opens a little but the 600 not at all
    7 grand is a little high i know but it took it surprisingly well my old man was reving it i was waiting for parts to start flying when i seen the tach

  6. #6
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    Quote Originally Posted by mrbigtrucks
    yeah the 750 opens a little but the 600 not at all
    7 grand is a little high i know but it took it surprisingly well my old man was reving it i was waiting for parts to start flying when i seen the tach
    the v belts did fly off though i didnt even notice my buddy was yelling to get my attention ops:

  7. #7
    Senior Member RACING JUNKIE
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    Heh, heh . . With apologies, You guys ought to be fun to watch.

    From a distance . . .

  8. #8
    Senior Member DYNO OPERATOR
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    No load no opening normal

    You must have a load on the engine at WOT like on a dyno to get the blades on the secondary to open. If you are free revving that engine to see this, you will NOT get the secondaries to open at all. The engine air demand has to be great enough to open them under full load and higher rpms than maybe 4000. You can not make a real mechanical double pumper very easily out of a vac secondary. We used to put a bolt in the linkage to make it open the secondaries about a 1/4" to quicken the opening of them. However a bog will likely follow. Sounds great with the air cleaner flipped! Everything Mark said as well in the earlier entry.
    Bruce

  9. Racing Junk


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