Here's s a snippet from page 1 (its actually 11 pages)
The 28AV and BV and 228-Series Carburetors
are of the single barrel, downdraft type with a
primary and a secondary venturi. The secondary
venturi is pressed into the primary venturi.
These carburetors are of the "balanced" design
in that all air for fuel bowl ventilation and
idling must enter through the air cleaner. For
this reason any restriction in the air cleaner
will have a minimum effect upon the fuel-air
mixture ratio. These models include single
idle, main, accelerating and power jets. The
accelerating pump is operated mechanically by
throttle movement through a link connected to
the throttle lever. The power jet is operated
by engine manifold vacuum.
The basic difference between the 28 and 228
Series is in the idle system. The 28 series
has a single idle discharge plug in line with the
closed throttle plate. The idle needle valve is
located in the air intake body, see Figure 3.
The 228 Series includes a two hole idle system
with one hole located above the closed throttle
plate and the other below the plate with the idle
needle valve located in the throttle body, see
Figure 3a,
