PRINCIPLES GASOLINE/DIESEL FUEL SYSTEMS - OD1620 - LESSON 1/TASK 2
(6) Secondary Air Valve (figure 32). Carburetors equipped with
mechanically operated secondary throttle valves are subject to engine
hesitation if the throttle is suddenly opened all the way at low engine
speeds, for the following reasons:
FIGURE 32. SECONDARY AIR VALVE OPERATION.
(a) The opening of primary and secondary throttle valves provides
too much venturi area for the engine to handle at low speed. It will not be
able to move enough air through the venturis to properly draw fuel from the
discharge tubes, causing a lean mixture.
(b) The secondary section of the carburetor is not equipped with
an accelerator pump system and will cause an engine hesitation at low speed.
To correct this deficiency, most carburetors with mechanical progressive
linkage use a secondary air valve. A secondary air valve fits into the
secondary throat and serves to restrict airflow through the secondary
venturi(s) until the engine is at a high enough speed to use them correctly.
The valve is actuated by a vacuum under it, which pulls it open against a