Demon 850 carburetor swap

Discussion in 'The Mixing shop.' started by Al68buickgs, May 10, 2017.

  1. Al68buickgs

    Al68buickgs Active Member

    I have a 455 big block in a 68 Buick GS with a 4. Speed I decided to Change out the Demon 850 Manuel carburetor for a Holley 750 electric chock. The mechanic recommended it will run so much better since I don't race the car and want a little bit more mileage for cursing nights. Notice high idle during first start stays high for a long time so I have to hit the gas padle to help idle down also driving and at stop she idle a little high too press gas paddle to lower rpm and suggestion?
     
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    The choke needs adjusting. The choke rotates a fast idle cam that boosts idle speed, so as long as the choke is engaged, it will boost idle speed. It sounds like the choke is staying on too long. If it is an electric choke, is it hooked up to key on 12 volts? There should be markings (lean/rich) on the black plastic choke thermostatic spring piece, and you should be able to rotate it after loosening some screws. Rotate it one notch towards lean and see if that improves things.
     
  3. DEADMANSCURVE

    DEADMANSCURVE my first word : truck

    fyi - be sure the (+) power is connected to a keyed ignition source not a constant source ( like to battery direct ) . also under the choke assy there is a set screw that runs against the cam he was mentioning . that screw controls the amount of RPM the choke is controlling . if the choke "timing" seems ok you can use that to adjust the RPM down . also check when possible to see if the throttle blade positioning is showing more than just a small square of the transition slot in the lower primary bores . you kind of have to remove carb to do that . its usually just about 1/2-3/4 turn after the throttle setting screw makes contact with tab on linkage .
    just thinking - was the demon carb manual choke or manual secondary's ?
     
  4. Al68buickgs

    Al68buickgs Active Member

    Demon carb manual ?
     
  5. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Manual choke.
     
  6. DasRottweiler

    DasRottweiler -BuickAddict-

    My Demon 850 has no choke, believe they call this version , the Mighty Demon. Mild cammed 462 idles at 700 rpm all day without a hitch. I know some folks aren't fond of them , don't see why........Jim/Rott
     
  7. DEADMANSCURVE

    DEADMANSCURVE my first word : truck

    yeah , i just had electric choke on my mind for some reason - plus some of the bigger carbs do have electric choke sometimes . he just stated it was "manual" so wanted to clarify if he meant choke or secondary .
     
  8. DEADMANSCURVE

    DEADMANSCURVE my first word : truck

    so , curious , how'd ya make out ? adjust etc the demon or get the holley ?
    and either way how's it running out ?
     

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