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Thread: Carb Problem

  1. #1

    Carb Problem

    I have a Speed Demon, #1402010, which is suppose to be a 750 cfm. When I put it on my street 454 and it seemed to drown it with fuel. I then switched to a Holley 650 cfm and it runs better. But I thought I'd get more power with the 750. Any suggestions? Here is my engine run down:

    454 block
    forged pistons - 10.75:1 compression
    new chromoly rings
    cam - .544 lift
    roller rockers
    rebuilt open chambered heads
    Weiand aluminum intake
    Mallory distributor
    Stock mechanical fuel pump

  2. #2
    Senior Member DYNO OPERATOR
    Join Date
    Aug 2007
    Lost Creek, WV
    There are a few things to look for. Since is seems to run ok with the 650 you fuel preesure should be alright. Check your float level. Then make sure your jetting is in the ball park for that carb. Do some research to find out the factory jetting should be. That will be a good starting point. Then check your power valve(s). They could be ruptured or wrong for your application.


  3. #3
    Senior Member RACING JUNKIE
    Join Date
    Mar 2008
    falkville al
    id be lookin at that 750 float level for starters.that engine should be able to handle 750.

  4. #4
    Senior Member SENIOR BUILDER
    Join Date
    Dec 2007
    Morrison, Colorado
    A larger carb doesn't always mean more power. Lots of things to consider. The biggest thing is calibration. It's got to be right for the motor it's on or it won't make a difference at all, and can even lose power. I'd take that carb completely apart and blow out all the passages, make sure there's no debris in it at all. Put it back together and set the floats with the level at the bottom of the sight glasses to start, and see how it does from there. What jetting, air bleeds, etc are in the carb? Did you get it used or new?
    Mark Goulette
    Owner/Driver of the Livin' the Dream dragster
    Speed kills, but it's better than going slow!!!!

  5. #5
    Senior Member DYNO OPERATOR bixblk's Avatar
    Join Date
    Dec 2004
    Chicagoland area


    That motor should like a 750 cfm carb.
    All the above info /tips I agree with.
    I have put 850 CFM DP holleys on 454's with similar parts, and it was fine.
    You probably have a carb issue with the 750
    Good Luck

  6. #6
    thanks guys.

  7. #7
    Senior Member MASTER BUILDER
    Join Date
    Mar 2008

    750 carb

    that holley 750 out of factory should have 75 front and 75 rear jets with a 6.5 power valve in both front and rear. no power valve in rear, then step rear jets up to around 82(and try 84s). you may have to step up the timing at idle to get the carb to idle properly(and remove some timing from the full advance position) art

  8. Racing Junk

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