fs3
I) Method of Operation
a) Vacuum piston actuation:
The vacuum piston (#1) is lifted by engine vacuum. A flexible rubber diaphragm (#2) is attached to the piston top and to the housing. This diaphragm divides the top part of the carburetor into a vacuum chamber (#3) above the diaphragm and an atmospheric pressure chamber (#4) below the diaphragm.
b) Atmopheric air pressure (approximately 15 Ibs/in2) passes through the air passage and occupies the space below
the diaphragm. A drilled hole in the
vacuum piston (#5) provides an unobstructed path up to the vacuum chamber.
As air is pulled past the vacuum piston by engine vacuum — creating a low
pressure area at this point — the piston
hole permits this low pressure to fill the vacuum chamber. Unequal air pressure
exists on either side of the dia
phragm.
Atmospheric pressure pushes up on the diaphragm, lifting the vacuum piston (#1) as well. The diaphragm (and piston) continue to lift until the vacuum piston return spring (#6) and the low air pressure match the pushing atmospheric pressure below.
c) Air speed through the venturi, which controls the
amount of low pressure in the vacuum chamber, is controlled
by a
butterfly valve (#7). A cable connects the throttle grip directly
to this butterfly valve. Twisting the grip
ooens the butterflv valve.