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Hole or No-hole?: 308 Weber DCNF 40 Venturi/choke Question...

Discussion in '308/328' started by ProvaMo, Mar 21, 2021.

  1. ProvaMo

    ProvaMo Karting
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    Jun 29, 2004
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    Paul John
    I've searched and I'm looking for insights: I'm rebuilding (& re-jetting) my 308 Weber DCNF 40's which had 34mm venturis in them.

    Pictured are OEM 32mm venturi's/chokes, with a groove and a cross-drilled 2.5mm holes, next to an aftermarket set of venturi's (which I bought from a well know and reputable Weber carb parts supplier) The new set does not have the grooves or cross-drilled holes.

    Here's my question with pictures: Does anyone know what the venturi groove and hole is for, and does it make a difference that the new set does not have these? My guess is maybe intra-carb air-bypass/correction(?)

    Thanks in advance for your inputs. Paul
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  3. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    No groove, nor hole, shown in the venturis (chokes) in any of the figures in the Haynes Weber Manual Chapter 13 covering 40DCNF. Additionally, even if present, there is no passage that such a groove/hole would "communicate" with in those figures. Perhaps, those same chokes are used in other Weber carb applications where they would have some function?
     
  4. ProvaMo

    ProvaMo Karting
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    Steve- Thanks for your reply, see the attached picture with markings. There's a corresponding hole in the primary 40mm carb bore which is aligned to the venturi groove landing area. I guess there's a possibility they serve a specific function for another configuration(?). The pictured carbs are off a 1979 USA 308 GTS, and believe they're original. The new 32mm venturis are for my late GTB Euro dry sump, I will check if that carb set has the corresponding holes and report back.
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  5. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    Can only say that that hole (and its function) is not disclosed, nor discussed, in the Haynes Manual -- but the Haynes Manual only covers thru 1977 US 308 (i.e., the 40DCNF 57/58/59/60 set). For the 1978-79 US 308, maybe there was a "refinement". If those carbs are off a 1979 US 308, are they the 40DCNF 72/73/74/75 set?
     
  6. ProvaMo

    ProvaMo Karting
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  8. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    Jan 11, 2001
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    Steve Magnusson
    Never noticed it on my ex-72/73/74/75 set, but also never had the chokes out. Did the 34mm chokes that you removed have the groove and holes?
     
  9. ProvaMo

    ProvaMo Karting
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    Jun 29, 2004
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    Paul John
    Steve- thank you...good call. I just checked my 1980 Euro Dry Sump car (that the "new" 32mm chokes are going into) and just confirmed that it has the 57/58/59/60 set of carbs. Attached is a picture of a 34mm choke that came out of it. Also, it does not have the main bore hole in the groove landing area that my 1979 US GTS has (which has the the original looking 32mm chokes) and the 72/73/74/75 set of carbs in the prior picture. Question still remains what the groove is for on the 72/73/74/75 set, but at least the new choke matches the dry sumps 57/58/59/60 set of carbs.
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