zenith carburetor mixture adjustment

[WPVT]

Would anyone know how the idle mixture needle screw works on a Zenith 2136016 (9786) carburetor? I am used to in/lean, out/rich, but I know that some older carburetors operate the reverse of this.

Thank you.

 

[Ivan Saxton]

I have just laid hand again on my very old and fragile 24 page book,  "Hints on Fitting and Adjusting the ZENITH Carburetter.   I am asking my son to scan, clean up, and adjust the size  so it is easier to use.   The publication is 1915,  so it should be useful for any instrument also well into the 1920s.   Some of the advice is probably also closely applicable to tuning other carburetters on antique automobiles for driving on modern roads with modern fuel.      ( In beautiful handwriting on the Title page, is the note that for a Lycoming 4,        135 Compensator jet,    120 Main Jet,    26  Choke,  ( which probably means a 26mm venturi  ID) ,    in a # 36 carburetter  ( which is probably 36mm flange throat diameter ).

When I have made bronze replica Zenith  105DC dual throat updraft Carbs for Stutz, Lancia, or Isotta Fraschini use,  I have ground the teeth off a big old flat file, and ground the profile of the venturi throat on the side, with relief under the top cutting edge.

    ( Almost any antique automobile restoration requires work that can only be done on a lathe:   So if you cannot find for yourself a good South Bend Lathe,  You will certainly find that an honourable, economical, oriental lathe with Imperial leadscrew will serve you well, and improve the way you deal with problems.)

[WPVT]

Thanks. Since posting I have learned that this updraft Zenith mixture needle threads in (clockwise) to enrich the mixture, and out (counter-clockwise) to make the mixture more lean. One and a half turns out is still the best place to start.