Hot Idle Issue Holley Carb

Discussion in 'Engine Topic' started by 71smalbloc, Aug 20, 2019.

  1. 71smalbloc

    71smalbloc Member

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    I'm having an issue where my engine (350) doesn't want to idle after about 30-40 mins of driving. When I come to a stop after about 30 mins of driving the car will idle normally at around 900 rpm for close to a minute but then the idle drops and the engine begins to stumble. If I crank up the idle screw it solves the issue but the car will idle way too high. Car runs and idles flawlessly for the first 30 minutes or so. I'm thinking this has to be a carburetor issue (Holley 650 recently rebuilt). Any ideas or recommendations?
     
  2. Zspoiler

    Zspoiler Veteran Member

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    Have you looked as heats shields and or thermo carb gaskets
     
  3. G72Zed

    G72Zed Veteran Member

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    Sounds like the carb was setup & tuned when not fully up to temp/heat soaked, and once it does come up to full temp/soak it's lean.

    Once this starts, the idle goes down, and so does your vacuum and timing, then leans out some more, then vacuum/timing again goes down and spirals till it stalls.

    Adjust your primary curb idle so as to expose the primary transfer slots just square, .020-.025 or so.

    Use a vacuum gauge or tach to set highest idle/vacuum with the idle mix screws, keep all 2 (or 4) the same. on a 2 corner idle setup, start at 2 turns out from slightly bottomed, on a 4 corner, you can go 1.5 turns out, it depends on a lot of variables, but this will get you going. Once this is done, turn down the curb idle to your desired RPM, if more idle air is needed, adjust the secondary throttle set screw, passenger side under the base plate, small flat, they are a little tuff to get at and turn.

    Make sure all tuning is done at 170-180* and intake at full temp, like after a 30 minute drive. A big fan in front helps move the air some.

    After your done, your engine will be a little on the rich side when cold, and correct when up to full temp, and will not stall if the carb is correctly built

    Hope this info helps you out.
     
    71smalbloc and BC_Homegrown like this.
  4. 71smalbloc

    71smalbloc Member

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    I'm running a 1" phenolic spacer to help with heat.
     
  5. 71smalbloc

    71smalbloc Member

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    This helps, thanks!
     
  6. G72Zed

    G72Zed Veteran Member

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    Would that be phenolic plastic/composite or wood laminate??

    The plastic/composite may be another issue once heat soaked, one of the main reasons they say for "drag" only, I use them for Dyno R&D only, then buy or make what it needs out of wood laminate or aluminum. Just my experience with those spacers.

    Your welcome, hope you get this solved.
     
  7. biker

    biker Veteran Member

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    This advice from G72Zed cured the exact same issue I was having. I had been bumping up idle speed screw as well, but all that did was make the idle mixture screws more ineffective. Dropped idle speed screw down, richened up mixture screws and it was perfect. Seems a bit counter-intuitive to add fuel to a hot engine, but the reasoning is sound when u think about it.
     

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