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Need help - carb sync - can't get an idle

Discussion in 'Technical Q&A' started by Mike328, Jan 28, 2004.

  1. Mike328

    Mike328 F1 Rookie
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    Oct 19, 2002
    2,655
    Boulder, CO
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    Mike
    Just got the carbs back on, trying to get the car to idle at some reasonable speed. Currently, it tries, and then just dies.

    All of the throttle plates are fully closed; I don't want to set the idle speed by opening the plates a little (as this affects their position with relation to progression holes).

    Let me just state, if this fixed throttle plate method fails, I'm more than willing to sync things up the more traditional way. That said...

    What do I need to do to, with throttle plates fully closed, to increase the idle speed? I've got air balance screws (currently fully closed) and idle mix screws (tried three turns out on all, then two turns--still nothing) to work with.

    --Mike
     
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  3. rudy

    rudy Formula Junior

    Jan 13, 2004
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    Los Angeles California
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    Rudy Hassen
    I thought the plates needed to be open a small amount. How would air/fuel get in? Are they webers? I haven't dealt with carbs in a LONG time. Are you following some factory procedure? Feel free to ignore this post if these are stupid questions.
     
  4. dm_n_stuff

    dm_n_stuff Global Moderator
    Global Moderator Lifetime Rossa Owner

    Dec 10, 2003
    36,953
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    Dave
    I have the weber book.

    If you'd like the pertinent pages, drop me a pm, I'll scan and email them to you tomorrow.

    Gotta be honest, I'm not terribly familiar with Webers, mostly worked on Brit cars with Strombergs, but I figure the book's gotta help some.

    Let me know.
     
  5. Mike328

    Mike328 F1 Rookie
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    Oct 19, 2002
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    I've got a few books I'm working with... The Haynes weber manual and then the HPBooks Weber Carburetors, as well as the Ferrari workshop manual, and also the Piere Manifolds Weber Tuning Manual.

    The issue is almost for sure, as pointed out, that not enough air is getting past the (fully closed) plates.

    I'm thinking opening up the air bleed screws, to feed some air into the throats AFTER the plates might work out.

    Note to future readers of this thread: I know I sound like an idiot with this fixed throttle plate thing... But maybe... Just maybe... It might turn out to be more like innovation and less like stupidity :).

    Off to open up the air bleed screws...
     
  6. Bryan

    Bryan Formula 3

    #5 Bryan, Jan 28, 2004
    Last edited by a moderator: Sep 7, 2017
    To my knowledge, all Webers (the only carb on production Ferraris) have an idle bypass circuit that lets some fuel and air through even when the plates are completely closed.

    Attached is the fuel flow in a Weber IDF carb. As you can see, the air bleed is one means of getting air into the carb downstream of the throttle plate. However, there should be air getting through the calibrated air bleed that is part of the idle jet circuit. I have never had to fiddle with the air bleeds (DCOEs).

    Were all the passages cleaned and the float levels adjusted when the carbs were out?

    I have some luck with DCOEs with the following procedure.

    Disconnect all the throttles rods so that all carbs run independently.

    Back off idle speed screw until it just barely contacts the lever arm that moves the throttle plate.

    Adjust idle mixture screws to 2 turns out (this may vary from car to car and your note indicates that 3 seems to work for you).

    Start car. On mine, this results in an idle speed of 900 rpm. If idle speed is too low, adjust idle speed screw SLOWLY to increase. If idle speed is too high, use idle mixture screws to reduce. I make the same small incremental adjustments on all the carbs. Once you get a reasonable idle speed, begin the normal synch process on individual carbs.

    Re-connect throttle rods, so that no change in carb setting occurs.

    You are correct that you need to get the idle about right with the throttle plates as closed as possible to avoid the progression circuit from coming in.

    My $0.02 worth. Good luck
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  8. Bob308GTS

    Bob308GTS Formula 3

    Sep 26, 2001
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    Aurora,IL
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    Bob Campen
    Mike, to keep the plates in the same position as the rebuilder set them and have the engine idle, you will have to turn the mixture screws out more, start at 3.5 turns out and try it, if it dies again go a half a turn more after each failed try until it will idle. Then let it warm up and start adjusting.

    Good luck
     
  9. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
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    Roseburg, OR
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    Hans E. Hansen
    Mike: TEMPORARILY turn in the idle speed screws just to establish a stable idle. Play with mixture, linkage, etc.

    Simply get the car running first. Then sync and fine tune. You'll eventually get back to your 'theoretical' perfect sync mode.

    I had to crank the idle WAY up at first. Mixture adjustment and a few other tweeks let me back the idle down. Only then could I properly adjust the system.
     
  10. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
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    Roseburg, OR
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    Hans E. Hansen
    And, BTW, since you have the air cleaner off, this is a super prime opportunity to play around. Just record your 'baseline' settings so you can return to a known state. But turn every screw you can find and see what happens. Even if it's wrong. It's a prime learning opportunity. (Gee, I turned that screw and the engine died. WTF??) You'll learn volumes about how the whole damn thing functions. The more you learn, the better you will be able to get it running correctly. I went for some seriously weird settings just to see. Then back to a more traditional approach. Throttle plates shut, air bleed screws out 5 turns. Air bleed screws shut, idle speed screws turned in. You name it.

    You will find it easier to go with very rich mix screws, as the car will run OK for the other adjustments. Lean will give you a misfire. Get it all working up to snuff and then optimize the mix.

    However, when I was done, I had some major league carbon fouled plugs. Had to change them for the final tune.
     
  11. Bryan

    Bryan Formula 3

     
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  13. bill308

    bill308 Formula 3
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    May 13, 2001
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    Mike,

    Preliminary adjustment on the idle speed screws is to first make contact with the throttle lever then open and additional 1/2 turn. For an initial mixture adjustment, I'd start by lightly seating the needle valve then back out 3-turns. Initially, all air balance screws should be closed.

    You must have air flow past the throttle plates for the engine to run. This air flow is what draws emulsified fuel/air from the progression holes to support running.

    The mixture setting on my original 40DCNF 7x carbs liked to be between 3-4 turns open. When I fitted a new set of earlier 40DCNF 4x carbs, they seemed to like to be about 3 or less turns open.

    The balance screws are really only to equalize air flow between the 2-barrels within a carb. One should always be closed. The lower flowing barrel is adjusted up to match the higher flowing barrel. Carb-to-carb adjustments are made at the linkage.
     
  14. GTO84

    GTO84 Formula Junior

    Dec 13, 2003
    562
    Bill 308 is correct. To idle the throttle plates need to be open. This is your problem, period! Do not play with every screw to see what happens. That is opposite of what you are trying to accomplish and only complicates things. As a carb beginner you need to buy/read the weber book and do the simplest possible procedures. Carb adjusting is not about opinions or "doing it a new better way" it is about following a procedure systematically and, if done properly, finishing with the carbs dialed in to a gnat's breath.
     

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