Weber_40DCN-20 Flaw Found | FerrariChat
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Weber_40DCN-20 Flaw Found

Discussion in '365 GT4 2+2/400/412' started by type26owner, Jun 3, 2022.

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  1. type26owner

    type26owner Rookie

    May 29, 2022
    10
    Martinez, CA USA
    Full Name:
    Keith Franck
    #1 type26owner, Jun 3, 2022
    Hi, My name is Keith Franck and I was asked to try and find the cause of the lean transition problem. And sure enough I found it. The throat diameter around the auxiliary venturi (AV) is 42mm. Subtracting the cross-sectional area of the AV derives an area of 38mm which is the problem. The airflow velocity is too slow for the main circuit to tip-in and that is the cause of the leanness. So I designed and made some shirts which slip around the AVs and they decrease the area to 32mm which matches the typical choke size. The fellow I made these for is delighted with his Daytona and the tractability is to die for now. Is anyone else experiencing this problem? I'll try uploading some photos.
     
  2. type26owner

    type26owner Rookie

    May 29, 2022
    10
    Martinez, CA USA
    Full Name:
    Keith Franck
    #2 type26owner, Jun 3, 2022
  3. daytonaman

    daytonaman Formula Junior

    May 1, 2007
    986
    Australia
    Full Name:
    howard pigdon
    #3 daytonaman, Jun 4, 2022
    maybe post this in vintage section for feedback
     
  4. type26owner

    type26owner Rookie

    May 29, 2022
    10
    Martinez, CA USA
    Full Name:
    Keith Franck
    #4 type26owner, Jun 4, 2022
  5. SouthJersey400i

    SouthJersey400i Formula 3

    Mar 14, 2007
    1,666
    Romulus, NY (Finger Lakes)
    Full Name:
    Ken Battle
    #5 SouthJersey400i, Jun 12, 2022
    The main venturi sets the airflow velocity. The aux. venturi has little effect on this airflow. Looking at your photo and your metal ruler, the main venturi looks like it might actually be less than 32 mm. What am I missing?
    Ken
     
  6. type26owner

    type26owner Rookie

    May 29, 2022
    10
    Martinez, CA USA
    Full Name:
    Keith Franck
    #6 type26owner, Jun 12, 2022
    The choke (Main venturi) is 32mm but the bore that the auxiliary venturi slides into is 42mm and that's why I added the scale in the photo. So by adding my shirt the same amount of air at a particular throttle opening is flowing past the AV but it is doing at a higher velocity. That increased velocity is needed to cause the AV to tip-in the main circuit at the proper time so the lean flatspot in the transition is eliminated. So you are right, this problem can be fixed by installing even smaller chokes but do you really want to run say 26mm chokes?
     
  7. SouthJersey400i

    SouthJersey400i Formula 3

    Mar 14, 2007
    1,666
    Romulus, NY (Finger Lakes)
    Full Name:
    Ken Battle
    #7 SouthJersey400i, Jun 13, 2022
    Understand now your issue was the transition. If your skirts work, so be it. I'm surprised they do not interfere with the main chokes at full throttle. Transition is usually handled in a Weber with little holes in the side of the main throat right at the point where the throttle plates close.
    I sure prefer the precision of Dellorto's vs. Weber's!
    Ken
     
  8. Ak Jim

    Ak Jim F1 Veteran
    Owner Rossa Subscribed

    Dec 23, 2007
    9,368
    North Pole AK
    #8 Ak Jim, Jun 13, 2022
    Those look really nice, good work! Are you working on these for any of the other Weber carbs? The 365GT4 2+2 and the 400s with the carbs use 38 DCOE.
     
  9. type26owner

    type26owner Rookie

    May 29, 2022
    10
    Martinez, CA USA
    Full Name:
    Keith Franck
    #9 type26owner, Jun 14, 2022
    This flaw stuck out like a sore thumb when I examined a carb the fellow sent me to look at. All the other carbs I've looked at including the DCOE and DHLA sidedrafts have similar cross-sectional areas for the airflow to pass through the auxiliary venturi and the throat obstruction of the butterfly shaft when at wide open throttle. Only on this carb did Weber screw the pooch. I'm just glad I was ab lo to find the flaw and fix it.

    I've reverse engineered the DCOE and as a result have come up with a far superior idlejet and VP tube which takes the place of the bubbling emulsion tubes. I've sold quite a few without a single customer returning my jetting or complaining. I also supply the VP tubes for the DHLA too.
     
    Tojo likes this.
  10. Tojo

    Tojo Formula Junior

    Apr 12, 2002
    497
    Sydney
    Full Name:
    Tim
    #10 Tojo, Jun 14, 2022
    Sounds interesting, I’m not far off re-kitting my DCOE’s. Do you have a website, or is pm the best way to get in touch?


    Sent from my iPad using FerrariChat
     
    Ak Jim likes this.
  11. type26owner

    type26owner Rookie

    May 29, 2022
    10
    Martinez, CA USA
    Full Name:
    Keith Franck
    #11 type26owner, Jun 15, 2022
    I updated my Personal Details which will guide you to my Sidedraft forum.
     
    Tojo likes this.

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