Race Demon 750 DP idle issues

Discussion in 'Engine Topic' started by C4Racer, Sep 7, 2019.

  1. C4Racer

    C4Racer Veteran Member

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    So I have a holley DP 750 race demon carb and it holds the idle high when driving and coming to a stop - this is a manual trans car - pop the clutch in when the motor is 1500rpm or so and it idles high - about 1350-1400. When you first start the car it idles at the set point which is 1100. It's a big cam. small block 406. I just changed the disty to a MSD with vacuum advance so I wonder if it's pulling in more timing at high vacuum levels at 1500-2000 on decel and then it holds it there. I could try blocking that to see if it changes.

    but assuming it's not related to timing - what would be causing the throttle to hang up on this carb? Car has a lokar style cable linkage - I haven't really checked that out too much either.
    Has a return spring. Doesn't seem to happen when I start the car and test it when stationary so I can see the throttle. Maybe it's just sticking a bit?

    Ideas?
     
    1980RS likes this.
  2. G72Zed

    G72Zed Veteran Member

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    From my experience, I have seen soft gaskets, or gasket stacking with some spacers (based on material) cause some binding issues once the carb base plate gets fully heat soaked. I have had to repair many "ears" that were broken or cracked from people using a 3/8 ratchet and over tightening the carb. Look at the 1900 (I think) Fel Pro if open type gasket is needed and easy snugging in a X pattern with 1/4 ratchet, even a nut driver does the trick, go easy and repeat at full temps.

    Also, fuel seepage and grit can work into the P&S shafts after a while. Good cleaning, soaking of the base plate, Brake clean and blow out with compressed air works well. If the shafts are not loose, Use a little oil on the ends of the shafts and work in manually, if does free things up. Check primary and secondary shaft springs to make sure the locations are correct and not to soft.

    If that does not help in the carb area, you may have some sticking weights, or the vacuum rod is not returning all the way.

    Hope this helps you out C4Racer.
     
  3. C4Racer

    C4Racer Veteran Member

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    It does thanks. I will try some of that and check the gasket carefully. This is a new spacer / gasket setup too.
     
  4. 1980RS

    1980RS Veteran Member

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    Is this a Mighty Demon with no choke horn? My Mighty Demons idle as low as 650 if I want them too. There is a procedure to setting up a demon for a good idle.
     
  5. C4Racer

    C4Racer Veteran Member

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    Right. It idles just fine but then sticks when coming off RPM.
     
    1980RS likes this.
  6. 1980RS

    1980RS Veteran Member

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    Ok, if it idles well then I would look for a vacuum leak. Do you sill have the power valve in it? I have had some carburetors that acted a little funny with them and big cams. Here is something else you can try, take off the sec. linkage and run it on the primary side only that way you can isolate the problem. Demons are a pretty will built unit. Pull of the base plate and close the throttles down and look to see if there is any light showing through them if there is loosen the screws slightly and shift them in to place so no light can be seen, I have had that trouble with Holley's in the past.
     
  7. C4Racer

    C4Racer Veteran Member

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    There is no vacuum leak. There are no vacuum lines other that the one to the disty. It doesn’t leak. All the rest are plugged. Also a vacuum leak does not cause a high idle on a motor like this. It makes it idle worse.
     
  8. C4Racer

    C4Racer Veteran Member

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    There is nothing even remotely in the way of the throttle arm, so it's not a mechanical thing. But the throttle shaft did seem rather dirty. So I shot it with some brake-clean and then WD40 and seems to be smoother and always goes cleanly back to position now at least when working it manually. Also shot some WD40 into the cable mechanism for good measure but I don't believe that was sticking.

    So we'll see if that solves it. Road test time!
     
  9. C4Racer

    C4Racer Veteran Member

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    that pretty much fixed it, but it still hangs up just a little bit every once in awhile. I'm thinking maybe a stronger return spring may solve that. All working pretty darn good right now - overall 90%+ so first world problems kind of thing!
     
  10. dcozzi

    dcozzi Veteran Member

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    Double return spring and attach the return springs straight towards the arc travel of the the linkage, not at an angle. Ideally, it would pull the opposite direction of the throttle cable.
    Mine used to hang up on the underside of the air cleaner. Good times. For awhile, mine was not idling right and an old Holley tuner told me to make sure the butterflies are closed so it is not between idle circuit and main circuit. 1980RS has a good point about starting with them closed and it is crucial point to reference tuning.
     

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