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Webber flat spot

Discussion in '308/328' started by Herb Kirshrot, Jun 30, 2007.

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  1. Herb Kirshrot

    Herb Kirshrot Rookie

    Feb 12, 2007
    24
    Portland, Oregon
    #1 Herb Kirshrot, Jun 30, 2007
    I have a flat spot in the carbs on my 79 GTS. My mechanic tells me that it is typical of these early cars. He suggests that every carb car has it but though the RPM's may vary. I find it hard ot believe that Enzo designed and accepted a flat spot into one of his cars, and I don't recall any problem when I test drove one in 1979. Help!!
     
  2. greg328

    greg328 F1 Rookie

    Nov 17, 2003
    4,203
    Austin, TX USA
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    Greg
    #2 greg328, Jun 30, 2007
    Not normal.
     
  3. Napolis

    Napolis Three Time F1 World Champ
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    Oct 23, 2002
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    Jim Glickenhaus
    #3 Napolis, Jun 30, 2007
    True. You can move the flat spot around but it's often there. There are many books on Weber's and there a few tricks but the flat spot is typical.

    The guru is Mike Pierce at Pierce manifolds.

    http://www.piercemanifolds.com/
     
  4. BillyD

    BillyD Formula 3
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    Feb 28, 2004
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    #4 BillyD, Jun 30, 2007
    At what speed/rpm is the flat spot? Turning?
    Bill
     
  5. Herb Kirshrot

    Herb Kirshrot Rookie

    Feb 12, 2007
    24
    Portland, Oregon
    #5 Herb Kirshrot, Jun 30, 2007
    Typically at 2,800 RPM's. Though it occurs often during throttle transition at higher RPM's in higher gears but not in first gear at all.
     
  6. BillyD

    BillyD Formula 3
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    #6 BillyD, Jul 1, 2007
    Sounds like the typical flat spot when the Weber goes from idle jet circuit to main jets. Can be cured by carb rebuild and/or jet change.
    My flat spot was caused by previous owner installing too big of main jets, once I went back to stock it was gone.
    Bill
     
  7. Ferraripilot

    Ferraripilot F1 World Champ
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    May 10, 2006
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    John!
    #7 Ferraripilot, Jul 1, 2007
    I had the same flat spot when I had stock jets. I kept my idle jets the same at .55, ran those a little richer, and then placed 200ac and 140 mains and no more flat spot. Check to make sure you ignition curve is correct as well.
     
  8. snj5

    snj5 F1 World Champ

    Feb 22, 2003
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    Russ Turner
    #8 snj5, Jul 1, 2007
    This does sound like the classic weber transition flat spot between the transition and main circuits or ignition advance

    I basically agree with everything said as well, would just also underline:
    Ensure ignition advance is well lubricated and not binding.
    Ensure current idle and main jets are clean. Dirty jets will act as smaller jets, no? :)

    Something I learned recently that was a big Duh! for me was that when these cars were factory jetted around 30 years ago, gasoline formulations were totally different, and probably every stock Ferrari jetting is now slightly wrong for today's fuels and needs be fine tuned. So saying the stock jetting is right, is now more probably wrong simply due to different fuel characteristics.
     
  9. Randy Forbes

    Randy Forbes Formula Junior

    Jul 14, 2006
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    Sports Cars Plus,LLC
    #9 Randy Forbes, Jul 4, 2007
    Last edited by a moderator: Sep 7, 2017
    Before getting too deep into the carbs, as mentioned above, verify the rest of the engine is absolutely in the highest state of tune. Valves, if an adjustment is required, and the ignition timing are of paramount importance.

    Next item on the list, is to make certain that each carb is actuated at exactly the same time. The airflow has to be exactly the same through each venturi at the same moment. I've found the CarbTune (shown below) makes easy work of this task.

    Then, play with one (1) circuit at a time; slow run/idle jets, accelerator pump jets, etc.

    Since your flat spot sounds like it's happening during the transition from slow run to main circuit, sometimes a step ot two larger in accelerator pump jets will "mask" it. Might also require a "quicker" action of the accelerator pedal to get the required squirt into the cylinders.

    The slow run circuit is where I'd turn my attention next, again, working a size or two at a time.

    Chances are, once you get the engine's state of tune and the carb synchronization optimized, the flat spot will have diminshed enough to ignore.

    Also shown below, are the components that make up the individual circuits of the Weber carb; 45 DCOE sidedraft shown, but downdrafts are similar.
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