High idle in Park/Neutral, low idle in Drive

Discussion in 'Engine Topic' started by TheJuggernaut, Apr 12, 2012.

  1. TheJuggernaut

    TheJuggernaut Member

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    Feb 27, 2012
    Johnston, RI
    Hi Everyone! I am a long time reader and first time poster! After using the info I learned off this site and a good number of techbooks I took it upon myself to rebuild my '76 350 small block. Everything since break in has been going decent, but I have a small issue I am concerned with.

    In Park or Neutral it idles up at 1500. But when I drop it into drive it goes all the way down to around 600. This seems like an excessive drop to me. I had to set the curb idle high so that when it went into gear it wouldn't stall out. That being said, it runs fairly nice, I just hate when i go to park and the thing revs way up!

    Here are the specs:
    355, .030 over, Holley 670 Street Avenger, TH350, Stock heads, Edelbrock Performer Intake, Slightly larger cam than stock,

    It gets around 11 in/Hg at idle, vacuum gauge doesnt seem to jump around much, just slight twitches around the 11 mark.
    Timing is set at 13 degrees.

    Thanks in advance for any advice!!
     
  2. 73Camaro383

    73Camaro383 Veteran Member Lifetime Gold Member

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    Oct 29, 2010
    texas
    Adjust your mixture screws and lower the fast idle for the choke.
     
  3. Damon

    Damon Veteran Member

    12,847
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    Nov 16, 2000
    Philly area
    Check the timing both in and out of gear. If the timing is falling way back in gear, that's most of your problem. Usual culprit is a centrifugal and/or vacuum advance kicking in at idle. As soon as you throw it in gear it drops back, making the RPM drop worse than it would ordinarily be.

    Also check for vacuum leaks. They play hell with getting a reliable, stable idle.

    And, as was said above, make sure your idle mixture screws are set for best idle.
     
  4. TheJuggernaut

    TheJuggernaut Member

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    Feb 27, 2012
    Johnston, RI
    Thanks for the input. I'll check the advance springs and time it again. As for the fast idle, is that adjustable? I thought that it was either on or off.
     
  5. my24ktrat

    my24ktrat Veteran Member

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    Jul 5, 2007
    Roch., NY
    ...Also check to see if your vac advance is connected to full vac. port or timed port, sometimes connecting to full vac at baseplate will cause those symptoms, I usually run timed port on side of primary block, hope that helps, DC :bowtie:
     
  6. 73Camaro383

    73Camaro383 Veteran Member Lifetime Gold Member

    2,296
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    Oct 29, 2010
    texas

    There should be a fast idle screw somewhere. My eddy has one, I'm sure your holley does as well. Its only for the choke portion. Once shes warmed up it shouldnt be a factor though.. Might not be your issue but something to check at least
     
  7. I had the same issue - exessive idle speed just to keep it running in gear.
    culprit was the centrifugal advance and some carb issues.

    first check that your distributor is working correctly and the timing isn't changing for the fast idle (ok maybe fast idle can have slight advance) and idle in P and D.
    then one step at a time, read your vacuum gauge check the timing and set your best idle.
    it took me about 2 weeks to get it.

    your looking for no ignition chance between P and D
     
  8. 79powershifter

    79powershifter Veteran Member

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    Apr 22, 2009
    Southgate Mi
    Vacume leaks will cause that too
     
  9. TheJuggernaut

    TheJuggernaut Member

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    Feb 27, 2012
    Johnston, RI
    Ok, just took a look at the timing. In Park or Neutral it is at 16 degrees. When I shift to Drive, down to 2 degrees! The fast idle screw on my carb doesnt seem to engage unless the choke is on full, so I don't think that is affecting it. I'm going to look at the advance springs, see what kind of shape they are in.
     
  10. Hoot

    Hoot Veteran Member

    1,477
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    Jul 12, 2008
    Greenville MS
    I would set timing at 16 idle(at least to start). With vacuum advance unhooked and plugged off. WIth idle set as low and you can and not die. THat way you shouldnt be getting centrifical advance yet. Then hook back up the vacuum adcance and turn idle to around 900-1k.

    Then look for vacumn leaks around the carb.

    Run the vacuum advance off the manifold vacuum port on the carb. not the ported. I know eddy carb has a manifold vacuum port on the carb base. Not sure on the holley though. been awhile since i ran one sorry.

    Now the above is after you make sure the distributer weights and springs are in good shape (not sticking or flying open to soon).

    Really does sound like a vacuum leak but i would check all the above while your at it. It wont hurt to get it dialed in.
     

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