THERMOSTATIC COIL SPRING
BAFFLE PLATE
CHOKE
VALVE
SHAFT
CHOKE
VALVE
VACUUM PISTON
HOT AIR INTAKE
(FROM EXHAUST MANIFOLD)
ASf04020
Figure 4-20.--Automatic choke.
EXCESSIVE FUEL CONSUMPTION. Can
engine is stopped, the thermostatic spring cools and
closes the choke valve. This choke is adjusted by
result from a high float level, a leaky float, a
loosening the screws holding the cover of the
sticking metering rod or full power piston,
thermostatic coil, turning the cover either right or left,
sticking accelerator pump, and too rich an idling
and retightening the screws. One trouble encountered
mixture.
with this choke is that carbon from the exhaust causes
A SLUGGISH ENGINE. May be the result of a
the vacuum piston to stick; as a result, no choking
poorly operating accelerator pump, sticking
occurs when the engine is cold.
high-speed piston, low float level, dirty or
In figure 4-21, the thermostatic coil is mounted in
gummy fuel passages, or a clogged air cleaner.
the manifold and connected to the choke valve by
mechanical linkage. A vacuum diaphragm is mounted
POOR IDLING. Often characterized by
on the carburetor and attached to the linkage to retard
stalling of the engine, usually is due to a too-rich
choking action after the engine is started. In the
idle mixture, a defective choke, or an incorrectly
operation of this automatic choke, the thermostatic coil
adjusted idle speed screw at the throttle valve.
closes the choke valve until the engine is started. After
FAILURE OF THE ENGINE TO START.
starting, the vacuum diaphragm, in conjunction with
May be caused by an incorrectly adjusted choke,
the air entering the carburetor throat, overcomes part of
clogged fuel lines, or air leak into the intake
the tension acting on the choke valve from the
manifold.
thermostatic coil, and lessens the choking effect. As the
engine warms up, the thermostatic coil continues to
HARD STARTING OF A WARM ENGINE.
expand and open the choke until operating temperature
Could be due to a defective or improperly
of the engine is reached. This type of choke can be
adjusted throttle link.
adjusted by two methods. Some manufacturers allow
SLOW ENGINE WARM-UP. May indicate a
for adjustment by providing more than one attaching
defective choke or defective radiator thermostat.
point of the linkage to the thermostatic coil, while
others provide adjustment by bending the mechanical
CHOKE VALVE
linkage between the thermostatic coil and the choke
VACUUM
DIAPHRAGM
shaft. Be sure to check the manufacturer's recom-
UNIT
mendation before adjusting this type of choke.
DIAPHRAGM
ROD TO
THERMOSTATIC
Carburetor Troubles
COIL
ENGINE
MANIFOLD
VACUUM
Here are some of the engine troubles that usually
(but NOT always) can be traced to some fault in the
ASf04021
carburetor system:
Figure 4-21.--Automatic choke with thermostatic coil
mounted in manifold.
4-13
