PRINCIPLES GASOLINE/DIESEL FUEL SYSTEMS - OD1620 - LESSON 1/TASK 2
shaft, so that the secondary throttle valve(s) will open all 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 with primary throttle operation when the choke lockout is engaged.
(b) Vacuumoperated secondary throttle valve(s) (figure 31) are
actuated by a vacuum diaphragm whose vacuum source is the primary venturi.
The principle of operation is that as engine speed increases, the vacuum in
the primary venturi also increases causing the diaphragm to pull the
secondary throttle valves open. There is linkage between the primary and
secondary throttle valve(s) in relation to the primary throttle valves.
FIGURE 31. VACUUM PROGRESSIVE LINKAGE.
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