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 TM 9-8000
vacuum diaphragm that operates the needle valve
temperature increases, opening the valve. This will in
through a fulcrum. The diaphragm is operated by a
turn admit air under the throttle valve compensating for
manifold vacuum. During periods of normal engine idle,
the overly rich mixture.
the manifold vacuum is not high enough to operate the
b. Throttle Return Dashpot (Fig. 4-59). The throttle
diaphragm and the needle valve remains open. During
periods of decelera- tion, the manifold vacuum is high
return dashpot acts as a damper to keep the throttle from
enough to cause the diaphragm to close the needle
closing too quickly when the accelerator pedal is
valve, shutting off the idle system. The needle valve also
suddenly released. This is important to prevent stalling
can be closed by pushing a button on the instrument
on cars equipped with automatic transmissions. The
panel that will energize a solenoid, closing the needle
throttle lever contacts the dashpot rod just before the
valve. The purpose of this manual actuation device is to
throttle valves close. This will, in turn, push in on the dia
clear the idle circuit and manifold of unburned gases
phragm. The diaphragm slows the closing of the throttle
before the engine is turned off.
because it must exhaust the air from the chamber
through a tiny venthole. When the throttle opens again,
the dashpot spring pushes the diaphragm back into
4-24. Accessory Systems. There are numerous
operating position, drawing air into the chamber.
devices that are used on carburetors to improve
drivability and economy. Their application varies from
c. Antidiesel Solenoid (Fig. 4-60). The anti- diesel
vehicle to vehicle. The following subparagraphs list the
solenoid controls the throttle opening at engine idle to
most popular of these devices.
prevent dieseling. Engine dieseling is a condition that
causes the engine to continue running after the ignition
a. Hot Idle Compensator (Fig. 4-58). The hot idle
switch is turned off. It is a particular problem with newer
compensator is a thermostatically controlled valve that
emission con- trolled vehicles due to higher operating
helps to prevent engine stalling when idling In very hot
temperatures, higher idle speeds, leaner fuel mixtures,
weather. Long periods of engine idle cause an excessive
and lower octane gasoline (para 4-40). The solenoid is
amount of vaporization of gasoline in the float bowl.
energized when the ignition switch is turned on, causing
These vapors will find their way into the carburetor throat
the plunger to open the throttle to idle speed position.
and cause an overly rich mixture.
The hot idle
The plunger length is adjustable so that the idle speed
compensator consists of a bimetallic strip of metal
can be adjusted. When the ignition switch is turned off,
operating a valve that controls an air passage that ends
the solenoid is deenergized and the throttle closes tightly,
under the throttle valve. The bimetallic strip, which
cutting off the air-fuel mixture. This will keep the engine
consists of two pieces of dissimilar metal with different
from dieseling.
expansion
rates,
will
curl
upwards
as
the
Figure 4-58. Hot Idle Compensator
Figure 4-59. Throttle Return Dashpot
4-36
TA233421
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