What carbs do I need?
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What carbs do I need?

  1. #1
    Senior Member Kylemenz1's Avatar
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    Default What carbs do I need?

    I have a Holley 420 Mega blower with dual 750's (Holleys blower carbs with no choke) on top of a 454. The motor is Mercury 525sc that I put the bigger blower on. I can't seem to get enough fuel through them to run 6lbs of boost. They run great jeted at 90 primary and 96 secondary while running 3lbs of boost, but with that set up I have not received any performance gains.

    I have switched pulleys for 6lbs and it runs awesome with big gains but way to lean. I don't think I can put big enough jets in the carbs to get a rich enough mixture to keep the blower safe, nice, and cool.

    So my questions are: Do I need to go to bigger carbs? Witch carbs should I go to? Can I just keep tweeking the carbs I have to make them work?

    Any input will help. Thanks.

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    I have never run your type of blower but....2 750's should feed those low boost pressures just fine. I'd say up to about 15 lbs you would be just fine.
    Be sure it is not a fuel supply problem.

  4. #3
    Senior Member Kylemenz1's Avatar
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    Do you mean 15lbs of fuel pressure?

    I am useing a Holley Black electric fuel pump, but am only running 7.5 to 8 pounds of pressure.

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    Are you useing a fuel return system rather than a deadhead system?

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    Senior Member Kylemenz1's Avatar
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    I am not useing any fuel return system.

  8. #6
    ptz
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    Question

    how large is the feed line ?
    it might be acting like a main jet.

    Pie r squared...

    -10 or -12 to feed with a -6 or -8 return would be a good start.

  9. #7
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    Quote Originally Posted by Fiat48 View Post
    I have never run your type of blower but....2 750's should feed those low boost pressures just fine. I'd say up to about 15 lbs you would be just fine.
    Be sure it is not a fuel supply problem.
    LOL. No...you are good to 15 lbs of boost pressure.

  10. #8
    Senior Member Kylemenz1's Avatar
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    I am using a # 8 fuel line from the pump to the regulator and two #6 lines from the regulator one line to each carb.

  11. #9
    Earth Motors
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    the carbs are pleanty big enouf, your high speed air bleeds need to be changed ,you have a poor signal be cause of the size of the carbs, I think smaller air bleeds will riched it up ,then you can put close to stock size jets in or get some 600cfm carbs you will have a better air signal which will draw more fuel in , cfm does not mean more fuel just more air,I cannot type

  12. #10
    Senior Member Kylemenz1's Avatar
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    The air bleeds are 75's. Are they too high? They are the ones that came with the carbs when I got them (used).

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    A Return System Is Used For A Setup Such As Yours. The Fuel Pressure Is Set And Never Changes Where The Deadhead System Waits Until The Fuel Meets The Pump pressure . At That Time The Fuel Demand Could Be 15 Lbs causing a lean fuel situation
    Ex. with a return systemThere Is always A Flow Of Fuel To Supply The Carbs,THE Return System Is Used On Efi Systems.

    There Are Some Great Articles On This Go To Century Performance For One.

  14. #12
    Senior Member Kylemenz1's Avatar
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    Sorry. The high speed air bleeds are 34 and the idler's are 75.

  15. #13
    Earth Motors
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    call a carb shop and ask them what size they recamend and order them barry grant ,holley ,the carb shop , ect.

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