750DP dual idle system

Discussion in 'Holley' started by Kenny462, Jun 6, 2016.

  1. Kenny462

    Kenny462 Gold Level Contributor

    something has gone badly wrong with my 750dp.it will not idle under 1000rpm and wants to die even then. runs great at wot. idle screws are not responsive, several day ago I modified fuel inlet tube and needed cut the exisiting tube and replace with rubber fuel hose, hope I didnt get filings in fuel tank, also used non vented cap as before.used a vented cap. what should I look for? I am not a great carb guy so please be simple. Thanks.Ken
  2. moleary

    moleary GOD Bless America

    start simple and double check no vacuum plugs came off...
  3. Kenny462

    Kenny462 Gold Level Contributor

    could there be an internal vacuum leak? Ken.

    DEADMANSCURVE my first word : truck

    I would check any pre carb filters for "junk" . the small filters at carb inlets ( if still there ) should be spring loaded and if plugged could allow crap to sneak past . may have clogged up idle passages ? and maybe go back to original style gas cap and see if any dif .
  5. No Lift

    No Lift Platinum Level Contributor

    Fuel inlet tube? Is that right before the carb? Is gas trying to flood into the primary or secondary looking into the carb throats while keeping it running?(Wear safety goggles.)

    If so pull both of your needle and seats for the carb and clean them out. You probably got a small amount of debris stuck on the rubber part. Possibly damaged the seat but cleaning them is worth a try.

    Just count the turns of the big nut, after loosening the big flat blade screw, it takes to remove the N/S. Then just screw it back in the same number of turns and you'll be close. Start the car and if it is idling nice set the floats per Holley's instructions.

    DEADMANSCURVE my first word : truck

    I thought he was talking about fuel inlet into the gas tank ? 'cause he then mentioned the gas cap switch .
    and you can shoot some carb clean into the needle n seat without removing - take a 5/8" box end wrench and hold adjuster nut in place , take a good sized flat blade screwdriver and back out the lock screw above the 5/8" adjust nut , note the round gasket when you do remove it . spray some carb clean straight down into needle and seat opening , replace lock screw n gasket . snug down firmly . do all this while keeping the 5/8" nut in position . or you can mark it with something and just dbl check . it usually will not move too much unless it has been on there awhile and kinda attaches itself to the screw .
    edit : holley tech info pertaining to "idle" - check idle screw condition and setting , idle passages , needle n seat , float condition n function , gaskets , throttle valves . also : good basic tune ( plugs , wires , filters etc ) , compression , vac leaks , clean out air bleeds , ck idle speed , intake/exh valve condition .
    I know that is quite a few things but pick a few simple ones and go from there . i'm sure some ( like tune up ) can probably be eliminated . clean out air bleeds with a small diameter wire and some carb clean . transition slots on bottom of throttle bores should only show a very little when carb is at rest - a big cam etc will change that a little with higher rpm idle requirements which will in turn effect the idle .
  7. No Lift

    No Lift Platinum Level Contributor

    That's worth a try before you actually pull the N/S.

    DEADMANSCURVE my first word : truck

    just thinkin -
    - whats the carb part # ? on front of choke tower .
    - and its pretty easy actually to remove bowls and metering blocks without removing carb a lot of times . then you can blow out passages , check the floats n jets etc . just need 2 sets of front n back gaskets . maybe replace the pump diaphragms while you're there .
  9. pancher1

    pancher1 Well-Known Member

    Something happen with your vacuum advance? In my experience, the idle screws become unresponsive when the idle screws are turned too far and the throttle blades fully expose the transition slots.

Share This Page