Quantcast Figure 4-51. Mechanical Progressive Linkage Operation.

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TM 9-8000
Figure 4-51. Mechanical Progressive Linkage Operation.
whose vacuum source Is the primary venturl.  The
throttle valves provides too much venturl area for the
principle of operation Is that as engine speed Increases,
engine to handle at low speed. It will not be able to move
the vacuum In the primary venturi also Increases,
enough air through the venturis to properly draw fuel from
causing the diaphragm to pull the secondary throttle
the discharge tubes, causing a lean mixture.
valves open. There Is linkage between the primary and
secondary throttle shafts to limit the opening of the
(2) The secondary section of the carburetor Is
secondary throttle valve(s) In relation to the primary
not equipped with an accelerator pump system and will
throttle valves.
cause an engine hesitation at low speed. To correct this
deficiency, most carburetors with mechanical progressive
f.  Secondary Air Valve (Fig. 4-53).  Carburetors
linkage use a secondary air valve. A secondary air valve
fits into the secondary throat and serves to restrict airflow
equipped with mechanically operated secondary throttle
through the secondary venturi(s) until the engine Is at a
valves are subject to engine hesitation if the throttle
high enough speed to utilize them correctly. The valve Is
suddenly is opened all the way at low engine speeds for
actuated by a vacuum under it that pulls it open against
the following reasons:
spring or counterweight force.
(1) The opening of primary and secondary


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