Several refinements have been made in the Model A carburetor which simplify and add to its smoothness of operation, especially at low speeds.
The original carburetor was provided with a double Venturi made up in two pieces. (See figures 422 and 423 in the January, 1928, Bulletin.). These parts have been replaced by a longer single Venturi (See A, Fig 569), the narrowest part of which is 27/32” in diameter. Other changes consist in the addition of a secondary well which is screwed into the lower half of the carburetor as shown at “B” and from which the idling jet derives its supply.
A slight change has also been made in the angle of the throttle plate and the plate is now stamped No 18 1/2 instead of No. 20. The location of the cap jet in the lower half of the carburetor has also been slightly changed.
These changes necessitated using a different combination of fuel orifices, the parts being stamped as follows:
Main jet now stamped No 19.5 instead of No 20.
The compensator is stamped No 19 instead of No 18.
The cap jet is stamped No 21 instead of No 19.
The idling Jet No 11 instead of No 10.
The new idling jet is slightly shorter than the old one, the new jet being 3” overall—the old one 3-5/64.
Figures 564 and 568 show present design parts. Old-style parts are shown in the January, 1928, Bulletin. Never attempt to use old style parts in present design carburetor. While the parts look alike the fuel orifices in the new parts have been changed to secure maximum results.
The instructions in the January Bulletin regarding carburetor cleaning and trouble checking remain unchanged with the exception that the breaker point gap should be set at .018” to .022” and the spark plug gap .027”. It is very important that all of the plugs be adjusted to a uniform gap of .027”
From letters received it is apparent that some owners still do not understand that after a Model A engine has been run in, the dash adjustment should not remain open more than 1/4” turn except for warming up the engine. Owners should be instructed that it not only wastes fuel but it is even harmful to leave this adjustment open longer than necessary.