Main Metering System - Chevrolet Light Duty Truck 1973 Service Manual

M A IN D ISC HA R G E
N O Z Z L E
BOOST V E N T U R I
M A IN V E N T U R I
T H R O T T L E V A L V E
primary air horn, a vacuum break diaphragm unit, fast
idle cam, choke linkage and a thermostatic coil which is
located on the engine manifold. The coil is connected to
the choke valve by a rod. The choke operation is
controlled by a combination of intake manifold vacuum,
the off-set choke valve, thermostatic coil characteristics,
atmospheric tmeperature, and exhaust manifold heat.
The remote thermostatic coil located on the engine
manifold is calibrated to hold the choke valve closed
when starting a cold engine, air velocity against the off­
set choke valve causes the valve to open slightly, against
the torque of the thermostatic coil. When the engine is
started, manifold vacuum applied to the vacuum
diaphragm unit mounted on the carburetor air horn will
open the choke valve to a point where the engine will
operate without loading (rich) or stalling (lean). The
choke valve will remain in this position until the engine
begins to warm up.
Through application of exhaust manifold heat, the
thermostatic coil relaxes gradually until the choke valve
M A IN W ELL A IR BLEED
POWER PISTON V A C U U M

MAIN METERING SYSTEM

Fig. 1 7 -Main Metering System
POWER PISTON
M E T E R IN G ROD
POWER PISTON
SPRING
M A IN M E T E R IN G
JET
POWER PISTON
A C T U A T IN G L IN K A G E
LOWER ID LE A IR BLEED
is fully open. Opening of the choke valve is controlled by
air flow through the carburetor air horn, past the off-set
choke valve and manifold heat acting upon the
thermostatic coil. As the engine is accelerated during
warm-up, the corresponding drop in manifold vacuum
and low air flow against the off-set choke valve, allows
the thermostatic coil to momentarily close the choke,
providing a richer mixture.
During warm-up it is necessary to provide a faster idle to
prevent engine stalling. This is accomplished by a fast
idle cam which is connected by a rod to a lever on the
choke shaft. The idle screw on the throttle lever contacts
graduated steps on the fast idle cam to provide a faster
idle than normal, to prevent engine stalling during the
warm-up period. When the engine is fully warm and the
choke valve is wide open, the fast idle cam rotates so that
the idle screw rests on the low step on the fast idle cam
where normal curb idle is obtained. Some carubretors
have separate fast and curb idle air adjustment screws. If
the engine becomes flooded during the starting period,
E N G IN E FUEL 6 M - 1 3
C A L IB R A T IO N
SCREW
LIGHT DUTY TRUCK SERVICE MANUAL