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Weber_40DCN-20 Flaw Found

Discussion in 'Vintage (thru 365 GTC4)' started by type26owner, Jun 4, 2022.

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  1. type26owner

    type26owner Rookie

    May 29, 2022
    10
    Martinez, CA USA
    Full Name:
    Keith Franck
    #1 type26owner, Jun 4, 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
     
    readplays likes this.
  2. type26owner

    type26owner Rookie

    May 29, 2022
    10
    Martinez, CA USA
    Full Name:
    Keith Franck
    #2 type26owner, Jun 4, 2022
    readplays likes this.
  3. Edward 96GTS

    Edward 96GTS F1 World Champ
    BANNED Silver Subscribed

    Nov 1, 2003
    11,097
    #3 Edward 96GTS, Jun 4, 2022
    im more concerned about a hot start idle issue. after a mile ir so then runs perfectly again.
     
  4. MiuraP400

    MiuraP400 Formula Junior

    Feb 3, 2008
    951
    Arizona
    Full Name:
    Jim
    #4 MiuraP400, Jun 4, 2022
    I’ve never had to do that to get rid of a transition circuit issue. Ignition timing and possibly idle jets normally fix the issue. Glad it is working for you.

    in regards to the idle issue. Try going for a drive for about 45 min. Shut the car off and let it sit for ten minutes. Open the hood and take the top of the air filter off. Look down the throats, is there any fuel dribbling into the barrels? If there is you can probably hear the fuel boiling in the carbs. First check the timing, retarded timing puts extra heat in the engine. If correcting the timing doesn’t fix it then try lowering the float. You can lower it around an eighth inch or so with minimal effects to the tune. The goal is to stop the spill over when it boils.

    cheers Jim
     
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  5. type26owner

    type26owner Rookie

    May 29, 2022
    10
    Martinez, CA USA
    Full Name:
    Keith Franck
    #5 type26owner, Jun 5, 2022
    My expertise is with sidedrafts such as the Weber DCOE and Dellorto DHLA. I have also been asked to look into a few downdraft carburetors to see if something was amiss. Each of those carbs I made and captured the exact dimensions with a 3D modeling software program. What is a common feature between all those carbs is the obstruction of the airflow in terms of the cross-sectional area is nearly the same past or through the butterfly shaft in the throat at wide open throttle and the auxiliary venturi. Achieving the correct minimum airflow velocity through the AV to bring the main circuit into play is critical for the tractability to be close to what can be expected from an EFI solution. In this regard Weber screwed the pooch with this particular carb for some unknown reason.
     
  6. miurasv

    miurasv F1 World Champ

    Nov 19, 2008
    10,760
    Cardiff, UK
    Full Name:
    Steven Robertson
    #6 miurasv, Jun 5, 2022
    Last edited: Jun 5, 2022
    This was a problem on Miura triple downdraft carbs that was later solved by Weber on the triples used on the Ferrari 365 Boxer wasn't it?

    My father developed a fuel drainage system on his SV that cured the problem and prevented the fuel leaking into the cylinders causing bore wash.
     
  7. miurasv

    miurasv F1 World Champ

    Nov 19, 2008
    10,760
    Cardiff, UK
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    Steven Robertson
    #7 miurasv, Jun 5, 2022
    Last edited: Jun 5, 2022
    Were these carbs causing the problems stated on an early Euro Daytona?
     
  8. MiuraP400

    MiuraP400 Formula Junior

    Feb 3, 2008
    951
    Arizona
    Full Name:
    Jim
    #8 MiuraP400, Jun 5, 2022
    Living in Arizona I have seen fuel boiling in a wide variety of carbs. I can not say the carbs on a Miura are more prone to the issue than any other carb. I have found that many mechanics are afraid to set the ignition timing correctly. They require more advance than most mechanics are comfortable with. If I remember right the P400 takes 22 deg static which drops down to 18 deg for the SV with its revised cam timing. Most mechanics like 8 to 12 deg at idle. The duel point distributors also cause problems because the points can be out of phase so the timing on three cylinders is different than the other three. To check the timing properly you really have check the timing on cylinders 1,6,7, and 12. If they are set up correctly the timing will be the same on all of them.

    cheers Jim
     
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  9. Edward 96GTS

    Edward 96GTS F1 World Champ
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    Nov 1, 2003
    11,097
    #9 Edward 96GTS, Jun 5, 2022
    ^ and for daytonas?
     
  10. type26owner

    type26owner Rookie

    May 29, 2022
    10
    Martinez, CA USA
    Full Name:
    Keith Franck
    #10 type26owner, Jun 5, 2022
    Yes, as far as I know. The fellow that asked for my help now has a sweet running V12 Daytona. I make custom designed emulsion tubes for the sidedrafts and he bought my tubes to fix the same problem he had with his Dellorto DHLAs on his BMW IIRC. He was so delighted with those VP tubes he asked me to solve the problem on his Daytona too.
     
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  11. MiuraP400

    MiuraP400 Formula Junior

    Feb 3, 2008
    951
    Arizona
    Full Name:
    Jim
    #11 MiuraP400, Jun 6, 2022
    I have not seen fuel boiling on a Daytona. But I have only worked on a few Daytona’s.

    cheers Jim
     

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