Figure 4-50. Two Barrel Carburetor with Fixed Linkage.
of throttle valves when the first is about two-thirds open.
(4) Because the secondary section of the
The geometry of the linkage is set up so that as the
carburetor only operates at high speeds, a low- speed
throttle reaches the full open point, all of the throttle
and idle system are unnecessary. There are numerous
valves will be wide open. The purpose is to provide a
devices and systems of linkage that are used to make
carburetor that will have a venturi that is small enough to
the secondary section of the carburetor operate. They
provide good throttle response and fuel economy at low
will be discussed in the following subparagraphs.
speed, yet be large enough to allow the engine to
e. Progressive Linkage Configurations. A carburetor
perform well at high speed. The section of the carburetor
that operates at low speed is called the primary section
equipped with progressive linkage is designed so that the
while the section that operates at high speed is called the
accelerator pedal directly operates the primary throttle
secondary section. It should be noted that the primary
valve(s). There are two ways in which the secondary
section of the carburetor works throughout the engine's
throttle valve(s) are operated.
operational range. It should also be noted that the
secondary section of the carburetor has no choke,
(1) Mechanically operated secondary throttle
accelerator pump, low-speed, idle, or high-speed
valve(s) (fig. 4-51) are actuated by linkage from the
enrichment system. These systems are unnecessary In
primary throttle valve(s). The linkage is designed so that
the secondary section of the carburetor for the following
it will not be actuated until the primary throttle valve(s)
are approximately two-thirds open. The operating arm
on the primary throttle shaft is made to be approximately
(1) The secondary section of the carburetor Is
three times as long as the arm on the secondary throttle
locked out so that its throttle valve(s) will not open when
shaft so that the secondary throttle valve(s) will open all
the choke system is operational on the primary side of
the way during the final third of primary throttle valve
opening. The operating arm on the secondary throttle
shaft operates through a spring so that it will not interfere
(2) At the speed that the engine is operating
with the primary throttle operation when the choke
when the secondary section of the carburetor begins to
lockout is engaged.
operate, there will be no hesitation without an
acceleration pump system.
(2) Vacuum-operated secondary throttle valve(s)
(fig. 4-52) are actuated by a vacuum diaphragm
(3) Because the secondary section of the
carburetor only operates at high speeds, it does not have
to be jetted for two stages of operation and therefore will
not require a high-speed enrichment system.