Carb tuning

Discussion in 'Ranchero Tech Help' started by zerolash, Mar 29, 2019.

  1. zerolash

    zerolash In Third Gear

    Messages:
    118
    I got 79 original 302 (100k) with gt40p heads,voodoo cam, elbrock intake and 600 Holley. I set initial timing to 15 advance to about 30. The problem is when I turn the idle down past 1400 rpm it seems it’s not getting enough fuel and it dies. I can give some fuel and it idles back up to 1500. I am not familiar with carb tuning. I have not checked my vacuum bc I can get it to idle correct. I will see if I can make a video.
     
  2. cbolt

    cbolt In Overdrive SILVER MEMBER

    Messages:
    590
    Location:
    Palm Coast, FL
    Sounds like you need to start from scratch and the mixture screws. I think with the Holley you turn them both all the way in until you just feel them bottom out (be gentle!) and then back them both out 1 and a half full turns. Hook up the vacuum gage and start setting them. There should be a youtube vid that can walk you through it.
     
  3. zerolash

    zerolash In Third Gear

    Messages:
    118
    I have done that. It needs to be running and idling correctly before you can adjust vacuum. I think it definitely not getting enough fuel. I may just have to try and go with a bigger jets
     
  4. pmrphil

    pmrphil In Overdrive GOLD MEMBER

    Messages:
    879
    No. Jets don't control idle, the mixture screws do. One on either side if it's a Holley, some Motorcraft have them in the baseplate facing forward. Find them, back them both out one more turn than where they are and see how that works. Keep the timing light hooked up just in case the vacuum advance got mistakenly hooked up to a manifold vacuum source. It should be disconnected to correctly set idle mixture AND base timing.
     
  5. zerolash

    zerolash In Third Gear

    Messages:
    118
    Ok. I have been trying to set my initial to 15 and trying to screw the mixture screws out. I feel I got the mixture screws half way out and that just seems pretty far. I am running a stick variation of a mechanical fuel pump could it be possible it’s too small?
     
  6. 5.0 Chero

    5.0 Chero Bahumbug Staff Member

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    13,763
    Location:
    Prunetucky California
    no its not the fuel pump
    Check for a major vacuum leak or possibly blown power valve
     
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  7. Doc76251

    Doc76251 In Fourth Gear

    Messages:
    344
    Location:
    Virginia Beach, VA
    I second the vacuum leak theory. I went down the whole carb tuning road and while far from an expert I learned A LOT (it was 35 years since the last time I touched a carb). My biggest problem following fixing all the vacuum leaks and trannt fluid in my vacuum system was the secondary throttle valve was open A LOT and I couldn't get the idle down below 1200 RPM and it ran like crap because there was always too much fuel. I took the carb off and flipped it over to get at the adjustment screw and set the secondary plates so that they "Just" moved and then set the fast idle and curb idle so that the transfer slot looked like a square. Flipped it over mounted it up and she ran to where I could adjust the fuel/air screws. Now it idles like a dream.

    Cheers,

    Doc
     
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  8. 1965 Ranchero 66G

    1965 Ranchero 66G In Maximum Overdrive PLATINUM MEMBER

    Messages:
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    Location:
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    X three on vacuum leak.
     
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  9. zerolash

    zerolash In Third Gear

    Messages:
    118
    Ya sprayed around for vacuum leaks and the power valve is good. I pulled it out and checked it. The power valve would dump fuel in and that’s not the problem.
     
  10. 5.0 Chero

    5.0 Chero Bahumbug Staff Member

    Messages:
    13,763
    Location:
    Prunetucky California
    New carb or used?

    also what is the list number on the carb

    possible its a reverse idle circuit carb
     
    Last edited: Mar 29, 2019
  11. zerolash

    zerolash In Third Gear

    Messages:
    118
    It’s is used.
     
  12. 5.0 Chero

    5.0 Chero Bahumbug Staff Member

    Messages:
    13,763
    Location:
    Prunetucky California
    List number
     
  13. zerolash

    zerolash In Third Gear

    Messages:
    118
    Holley 4160 600cfm
     
  14. pmrphil

    pmrphil In Overdrive GOLD MEMBER

    Messages:
    879
    the list number is stamped on the front of the choke housing, ya know, where the flap is
     
  15. zerolash

    zerolash In Third Gear

    Messages:
    118
    80457-11
    0835
     
  16. pmrphil

    pmrphil In Overdrive GOLD MEMBER

    Messages:
    879
    Fairly new "universal calibration" 600. Really shouldn't be an issue with fuel starvation with that one, they are normally on the rich side. Maybe a stupid question, but have you verified that zero on the damper is actually when the piston is at the top? Small block Fords have 3 different timing locations.
     
  17. zerolash

    zerolash In Third Gear

    Messages:
    118
    Yes I verified with a TDC indicator. I will say that I had a fuel filter connected to it and it idled worse so have a straight fuel line from pump and it runs a little better. That’s one thing that made me think it may be starving for fuel at low rpm.
    I tried putting it in gear to see what it does under a load and it just quickly died.
     
    Last edited: Mar 30, 2019
  18. 5.0 Chero

    5.0 Chero Bahumbug Staff Member

    Messages:
    13,763
    Location:
    Prunetucky California
    have you set the floats?
    if yes do you have fuel in the front float bowl?
     
  19. zerolash

    zerolash In Third Gear

    Messages:
    118
    I set the floats to where they are even. Going to try and get a vacuum and re set my mixture screws and see how that does. Thanks for everyone’s help so far!!!
     
  20. aquartlow

    aquartlow In Maximum Overdrive

    Messages:
    1,289
    Location:
    Summerfield Florida
    As previously mentioned, a vacuum leak is one of the first things to check(if possible) and not just at the carb but have a look at all your vacuum lines/hoses(I have seen a leaking brake booster vacuum check valve as being a culprit), as is verifying ignition timing. You may want to try shooting a bit of carb cleaner into the 8 air bleeds just above the boosters then use compressed air to clear them, may just be a little debris causing the issue. If that doesn't do it and the carb has sat a while or you have limited history knowledge of this carb, I would look into getting a carb kit for it(Advance auto parts sells Holley kits) and open it up to verify there isn't trash and/or debris inside the carbs very small orifices. Hope this helps
     

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