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Thread: 80457S 600 CFM won't idle!

  1. #1

    Default 80457S 600 CFM won't idle!

    I just installed a refurbished Holley 0-80457S 600CFM carburetor on my 1968 Mustang 289 with Auto Transmission and I can't get it to idle properly. If I increase the curb idle to where it will stay running it is way to high rpm. As I decrease the curb idle to lower the rpm it stalls and engine dies. I am replacing a similar older model Holley 4bl that I had no problems with the idle. I have tried adjusting the air/fuel mixture as outlined in the instructions with no help at all. Can someone please help me?

  2. #2
    Join Date
    Dec 2009
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    Ellington, CT
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    Check the float level(s) and the fuel pressure. Mechanical fuel pumps of that era, typically output 9+ psi of fuel pressure.
    Holley suggests a fuel pressure regulator to maintain pressure at 5-7 psi. It's in the manual. Also, check the 'needle & seat' assemblies; it might have debris jamming it partially open.

    Ensure you're not exposing too much of the idle transfer slots at idle (under primary throttle blades - must remove carburetor to measure with wire). Common problem. Check your carburetor manual for correct exposure but it's usually not more than .025"-.030". If so, you'll need to open the secondary blades a little bit, in order to close the primary blades (to maintain desired idle speed). When the idle transfer slots are exposed too much, it renders the idle mixture screws useless.
    Holley has a good video pertaining to this:
    http://www.youtube.com/watch?v=b8ROj...7BFF0&index=49 (Holley Tech - "How To Adjust The Secondary Speed Screw" video)
    Then view the "Idle Mixture Screws" video. View all their carb tuning videos.

    A blown power valve (although very rare), can also render the idle mixture screws ineffective.
    There's a power valve testing procedure in this link, if you think yours is defective:
    http://www.holley.com/data/TechServi...wer_valves.pdf (end of page, at left)
    May God's Grace Bless You

    '78 BRONCO: 508" stroker, TFS heads, Dominator MPFI & DIS, cold air induction, dual 3" exhaust/Magnaflow mufflers, 100HP progressive dry direct-port NOS, A/C, Lentech Strip Terminator wide-ratio AOD, 3:1 Atlas II, modified Dana 44/60-lockers-4.10s, hydroboost/4-disc brakes, ram-assist/heim joint steering, Cage long radius arms, custom traction bars, 4" lift, 35" mud tires

  3. #3

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    Danny;
    Thanks for the reply. The fuel pressure is around 6psi. Took the carb off & checked/adjusted throttle plates as described. Also checked for blown power valve. Still having the same problem. The carb idles fine while warming up, but when choke is open & engine is hot, still cannot set curb idle. As I slowly increase curb idle to keep engine from stalling, opening throttle after every 1/8 turn, it will go from barely running to about 2000 rpm & stay there for about a minute or so & then idle down on it's own & start to die. If I increase another 1/8 turn or less it will go up to 2000 rpm & stay there. Have tried adjusting idle mixture screws in from starting point & this makes it worse. Tried adjusting out from (1-1/2 turn) starting point 1/8 turn at a time both sides equally & this seems to make no difference. As I stated above I am replacing an older model of the same Holley 600cfm carb. & I had no problems with idle at all. This carb was supposedly factory refurbished & came in the correct Holley box with instructions & parts in sealed bag which leads me to believe it was. I have worked on this thing for about 8 hrs. now & am about to throw in the towel. HELP PLEASE!!!

  4. #4
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    Dec 2009
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    Quote Originally Posted by dreamzhappen View Post
    The carb idles fine while warming up, but when choke is open & engine is hot, still cannot set curb idle.
    Seems like its running excessively rich.
    Have you removed the fuel bowls, and inspected the needle & seat assemblies?
    May God's Grace Bless You

    '78 BRONCO: 508" stroker, TFS heads, Dominator MPFI & DIS, cold air induction, dual 3" exhaust/Magnaflow mufflers, 100HP progressive dry direct-port NOS, A/C, Lentech Strip Terminator wide-ratio AOD, 3:1 Atlas II, modified Dana 44/60-lockers-4.10s, hydroboost/4-disc brakes, ram-assist/heim joint steering, Cage long radius arms, custom traction bars, 4" lift, 35" mud tires

  5. #5

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    Danny;
    No I have not. Trying to stay away from tearing apart a brand new carb. and having to buy rebuild kit. Just seems odd that the curb idle adjustment would go from barely running to 2000 rpm & stay there with less than 1/8 turn of the screw.

  6. #6
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    Dec 2009
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    Ellington, CT
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    Quote Originally Posted by dreamzhappen View Post
    No, I have not. Trying to stay away from tearing apart a brand new carb and having to buy rebuild kit.
    It probably has the blue non-stick gaskets, so just removing the fuel bowls isn't a big task (and certainly not "rebuilding").
    You would need to remove the fuel bowl to change jets anyway.
    May God's Grace Bless You

    '78 BRONCO: 508" stroker, TFS heads, Dominator MPFI & DIS, cold air induction, dual 3" exhaust/Magnaflow mufflers, 100HP progressive dry direct-port NOS, A/C, Lentech Strip Terminator wide-ratio AOD, 3:1 Atlas II, modified Dana 44/60-lockers-4.10s, hydroboost/4-disc brakes, ram-assist/heim joint steering, Cage long radius arms, custom traction bars, 4" lift, 35" mud tires

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