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Page 34
Test Number 21 and 22-_Fast !dle and Automatic Choke Adjustment
The purpose of the fast idle function is to maintain proper engine
speed during cold engine and warm-up operation.
Additional throttle
opening is needed with the automatic choke butterfly valve in a closed
or partially closed position to keep the engine running smoothly and
prevent stalling. The choking action provides a richer fuel-air mixture
during cold engine operation to compensate for poor fuel_atomization.
The
increased idle speed warms up the engine quickly which enhances fuel-air
atomization and mixing.
The fast idle adjustment screw seats on a cam which is thermostatically
controlled through the choke linkage.
As the engine warms, the choke opens,
the fast Idle cam drops to progressively lower steps and the fast idle
speed gradually approaches curb-idle.
On a fully-warmed engine, the fast
idle cam releases the fast idle adjustment screw completely.
Engine
Idle speed is then controlled by the curb idle adjustment screw.
Fast idle adjustment procedures differ among the various automobiles.
Consult your specific vehicle service manual for proper procedures.
It
may be necessary to bypass one or more spark timing or emissions system
controls, either vacuum or electric, while adjusting the fast idle.
Many late model vehicles use a full electric or an electrically
assisted choke.
It contains a small electric heater which activates a
bimetallic coil spring or belleviile washer to reduce the choke time
under certain conditions.
This heater may be controlled by a temperature
switch located either in the choke housing or elsewhereion the engine.
The Ohmmeter Function can be used to confirm switch continuity, heater
element resistance and related wiring.
The Voltmeter function is used to
confirm the presence of voltage to and within the choke system as directed
by your vehicle service manual.
Use the RPM function to set fast Idle
per manufacturer's instructions.
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