idle too high, makes undriveable

Fuel and Air Systems

  1. phaelax

    phaelax Well-Known Member

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    Finally got some time to figure out where the gas was leaking with the new carb, it was the new dual feed fuel line. It's a quick fuel 750 with electric choke. The carb's instructions say the fast idle speed should be around 1600rpm, which it is. But as the car warms up, it gets faster and ends up idling at 2k when I expected it to go lower once it warms up.

    Drove it around the block a few times and it's nearly undriveable. With the idle so high, it's harder to control and I'm really having to mash on the brakes to keep from launching at stop signs. But then it's also misfiring if I try to floor it. Too much gas? Not enough? Do I adjust the primaries or secondaries?
     
  2. khuebner250

    khuebner250 FABO Gold Member FABO Gold Member

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    Vacuum leak?
     
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    • toolmanmike

      toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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      Yep, something is messed up. The fast idle cam should be free and you should be able to tap the gas and kick it off fast idle.
       
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      • harrisonm

        harrisonm Well-Known Member

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        I guess it could possibly be a vacuum leak, but I think not. Vacuum leaks will usually keep an engine from idling at all. I would say it was definitely carb related. I am not a Holley expert (in fact I don't like them at all), but I would say you have one of 4 things going on:
        1. Choke fast idle circuit not disengaging when warmed up. Find the fast idle cam and ensure it is completely disengaged when warm. It could be a problem with the fast idle screw, or the choke linkage travel may need be adjusted a bit so that the fast idle circuit will disengage when warm.
        2. Choke adjustment. Kina covered in #1. The choke linkage travel must be adjusted so that the choke plate closes when cold (you will, of course have to hit the accelerator linkage one time on a cold engine to set the choke) and opens fully when open. The choke linkage MUST also disengage the fast idle linkage.
        3. Mechanical idle screw and/or idle mixture screws set so poorly that the engine is idling at 2K.
        4. It is possible that the secondaries are partially open. I had that happen on a Holley several years ago and it resulted in a very high idle. Look down the carb throat to see. It's been SO long since that happened, I don't remember what I did. I seem to remember using some carb spray and fiddling with the linkage.
        Now don't take this wrong, but how much do you know about carbs, carb linkage and choke operation? If you don't know anything about how the choke and the fast idle linkage works, you will need some help. If you do understand it, you should be able to fix the problem pretty easily. I can almost guarantee that it is one of the 4 things above.
         
      • sport74

        sport74 Well-Known Member

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        Most definitely check for vacuum leaks. Just had a similar situation, but I'm running a race carb with no choke. First start up we idled it up to 2500 rpm which didn't take much adjustment. After break in, I tried totadjust the idle to set point. It wouldn't go below 1200 and that's when I heard what I thought was just air going into the carb. I got to searching and found a 3/8 Barb I had overlooked that was open. I put my finger over it and the idle dropped down to 3 - 400 before it died.
         
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        • phaelax

          phaelax Well-Known Member

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          Nothing really. Just what I can read from a book or find online.

          I'll check for vacuum leaks, but I don't think that's it. All the barbs came capped when I bought it, so the only ones missing are what I removed to connect hoses.
           
        • sport74

          sport74 Well-Known Member

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          Could be a leak elsewhere, use carb cleaner to gently spray around the intake everywhere that could leak, vacuum ports, intake runner plugs, carb mating area, even intake gaskets. Anywhere air could get in is a possibility of a vacuum leak. The idle will change as you spray an area with a leak. I have done this several times with starting fluid, but that is very dangerous, one stray spark and whoosh it's on fire! Carb cleaner is no where near as volatile and therefore safer
           
        • toolmanmike

          toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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          You never mentioned that you tapped the gas to kick it off the fast idle. You should be able to do that at any time after the engine fires. I usually let mine warm up a few min. first. And yes the idle will increase as the engine warms up. You should take off the air cleaner and start the engine cold and watch everything that the choke and fast idle cam does. Once you understand how it works you will be able to fix it.
           
        • Bulldozer

          Bulldozer free ice cream sandwiches

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          also make sure your throttle cable isn't adjusted so that it's not letting the butterflies close completely rendering the idle screw useless
           
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          • dano

            dano Evil Handy Man

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            Disconnect all vacuum lines and plug to isolate. What is the timing set to? Some Holley based carbs have a manually adjustable secondary to allow more air at idle without opening the primary too far and exposing the transfer slot. What model is the carb? Have you looked down the carb with it running to see if fuel is leaking or running down? Floats could be too high and or too much furl pressure. Electric choke is wire up and working? History of carb?
             
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            • robert flippo

              robert flippo Well-Known Member

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              I have a 600 Holley on a 318 with high rise, i was having same problem, at the back of carb was a vacuum line, i guees it was for power steering which i don't have, i capped that hose connection off and went from about 1800 rpm at idle down to about 800.But sounds like either a vacuum leak or hose not connected, possible fast idle linkage needs adjusted
               
            • famous bob

              famous bob mopar misfit

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              SOUNDS LIKE TIMING OR A VACUUM KEAK. DID IT EVER BACKFIRE ? COULD HAVE BLOWN A CAP OFF A VACUUM LINE .
               
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              • phaelax

                phaelax Well-Known Member

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                Brand new carb from Jegs:
                Quick Fuel SL-750-VS: 750 CFM 4 BBL Elect. Chok | JEGS

                I haven't had a chance to try anything since last time I posted so nothing new to update yet. And with just an unenclosed carport, I doubt I'll be working on it in this weather any time soon.
                 
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