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308 Tuning problem

Discussion in 'Technical Q&A' started by caponome, Mar 8, 2010.

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  1. caponome

    caponome Rookie

    Dec 30, 2009
    34
    #1 caponome, Mar 8, 2010
    I finished balancing my carbs as per Birdman's tutorial (excellent piece of work). I did not fine tune the idle mixture screws except to open one of them slightly on a carb that was spitting at about 4500 RPM, otherwise the mixture screws are at 5 turns and the carbs are synched nicely. The timing advance is 9 dgs BTDC at 900 RPM.

    When the engine hits 5500 RPM, as I increase the gas, the tach drops back to 5000 RPM and then as I continue to give it more gas the tach winds back up to red line. Any help on this would be appreciated, I'll be back at it this coming weekend. Thanks :)
     
  2. Sledge4.2

    Sledge4.2 F1 Rookie

    Oct 19, 2007
    4,789
    Marin
    Full Name:
    Geno
    #2 Sledge4.2, Mar 8, 2010
    someone that knows far more than i do can chime in, but i dont think the idle mixture screws would have any effect at 4500rpm...

    man there are alot of carb popping threads going on around here...
     
  3. FandLcars

    FandLcars F1 Rookie

    Aug 6, 2006
    3,057
    Tempe, Az
    Full Name:
    Rick Schumm
    #3 FandLcars, Mar 9, 2010
    You may find a lot of carb threads and info by using the search function. It also may be a good idea to post your question in the 308/328/Mondial forum, if you haven't already. I really have little experience with real carb tuning, but perhaps a carb cleaning and overhaul would help. Finding a good Ferrari tech in your area can be a real benefit for some of these tricky problems. Best of luck!
     
  4. caponome

    caponome Rookie

    Dec 30, 2009
    34
    #4 caponome, Mar 9, 2010
    From Birdman's tutorial and as I followed it through the detail views of the carbs in the WSM, I understand that the Idle mixture screws do little after the first progression hole, except maybe add a relatively small amount of fuel.

    After reading many of the Carb threads I was thinking that maybe the fuel mix went lean at 5500 and as I increased the throttle and exposed more progression holes enough fuel was fed to allow the engine to accelerate once more. It might be a benefit to up the main jets if this is what's happening, I really am just guessing but while my tach is moving backward I am increasing the throttle.

    I did a some work on the carbs, replacing acc pump diaphrams and cleaning the jets. My Idles are .55 and my mains are .125. Thanks again to anyone who can offer some help.
     
  5. tommott77

    tommott77 Formula Junior

    Feb 1, 2009
    652
    NC
    Full Name:
    Tom
    #5 tommott77, Mar 10, 2010
    I have been working on a very similar issue with my 308. The general consensus on the thread that I started on the subject was that it was probably ignition related (dizzy not advancing correctly) as opposed to fuel. I also had a conversation with Mike Pierce at Pierce Manifolds (well regarded Weber expert here at F-chat and fellow carbed 308 owner) and he felt that if it was a fuel issue it would be more of a fuel feed problem (fuel pump, clogged fuel filter/bad lines, float height). He did say that the one bit of tuning that could potentially make a difference over 4500rpms would be the air corrector jets. He said that my 220s were way off and 180s would be more appropriate. I have since decided to rebuild my carbs all together, so it might be a week or so before seeing if this cures the ailment.... If not I will be moving to the ignition and making sure that everything is working correctly there.

    If I find something I will try to update, and if you find something it would be appreciated if you could share as well. As mentioned earlier it seems a lot of folks seem to be running into similar ailments these days.
     
  6. caponome

    caponome Rookie

    Dec 30, 2009
    34
    #6 caponome, Mar 11, 2010
    I will be back at it this weekend and will certainly let you know what I find, thanks for your input. I did pull the output of the fuel pump and the flow seems pretty robust, but I didn't actually measure it.

    What puzzles me is that after dropping back to 5000 rpm, the engine recovers and progresses smoothly to red line. If the dizzys are the problem, why would they respond differently the second time the engine reached 5500 rpm? Never the less if I get an extra pair of hands I will try to measure the timing curves of each dizzy to see if they are consistant. Thanks again.
     

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