Theory Of Operation; What Is A Carburetor; Purpose Of A Carburetor - Chevrolet Light Duty Truck 1973 Service Manual

MODEL 4MV (4 bbl.) QUADRAJET
CARBURETOR
The Model 4MV carburetor has many features including
the following:
1. All models are calibrated to assist in meeting
emission requirements.
2. The highest step on the fast idle cam length
improves choke engine operation during the warm­
up period.
3. An exhaust gas recirculation system is used to
control oxides of nitrogen emissions. Dual punched
ports are located in the carburetor throttle body
bore to supply a vacuum signal for operation of the
exhaust gas recirculation valve.
Two punched ports, one just above the throttle valve and
one mid-way between the throttle valve and upper
surface of the throttle body are located in the primary
bore.
As the primary throttle valve is opened beyond the idle
position, the first vacuum port for the E.G.R. system is
exposed to manifold vacuum to supply a vacuum signal
to the E.G.R. valve. To control the vacuum signal at the
Theory of Operation...................................................... 6M-3
What is a Carburetor................................................ 6M-3
Purpose of a Carburetor........................................... 6M-3
Atomization................................................................. 6M-4
Metering....................................................................... 6M-5
Air Measurement.................................................... .6M-5
Fuel Metering.......................................................... .6M-6
Venturi Principle......................................................6M-7
Distribution...................................................................6M-7

WHAT IS A CARBURETOR

A carburetor is a metering device which mixes fuel with
air in the correct proportion and delivers them to the
engine cylinders as a combustible mixture. The design of
a carburetor is based on the application of natural
principles to the job of providing compatible air-fuel
mixtures to meet exhaust emission standards and
driveability for the varying engine requirements. Just as
it is necessary to understand the range of fuel mixtures
required for each operational phase, so must the
serviceman basically understand the natural forces
applied by the design for delivery of these mixtures.

PURPOSE OF A CARBURETOR

The purpose of a carburetor on a gasoline engine is to
meter, atomize, and distribute the fuel throughout the air
lower port, the upper port bleeds air into the vacuum
channel and modulates the amount of vacuum signal
supplied by the lower E.G.R. port. In this manner, the
E.G.R. valve can be timed for precise metering of
exhaust gases to the intake manifold dependent upon
location of the ports in the carburetor bore and by the
degree of throttle valve opening.
As the primary throttle valve is opened further in the
port throttle range at higher air flows the vacuum signal
decreases at the lower ports. At this time the upper port
ceases to function as an air bleed and is gradually
exposed to manifold vacuum to supplement the vacuum
signal at the lower port and help maintain correct E.G.R.
valve position.
The upper and lower vacuum ports connect to a cavity in
the throttle body which, in turn, through a passage
supply the vacuum signal to the E.G.R. tube pressed into
the front corner of the throttle body. The tube in the
throttle body is connected by a hose to the E.G.R. valve
located on the engine manifold.
The E.G.R. valve remains closed during periods of
engine idle and deceleration to prevent rough idle from
excessive exhaust gas contamination in the idle air/fuel
mixtures.

THEORY OF OPERATION

INDEX
flowing into the engine (Fig. 1). These functions are
designed into the carburetor and are carried out by the
carburetor automatically over a wide range of engine
operating conditions, such as varying engine speeds,
load, and operating temperature.
The carburetor also regulates the amount of air-fuel
mixture which flows to the engine. It is this mixture flow
regulation which gives the driver control of the engine
speed.
Regardless of engine speed or load, the carburetor must
automatically perform its three basic functions.
The automotive carburetor is an intricate device;
however, when studied one phase at a time, the functions
of the carburetor are easily understood.
As mentioned above, the three main functions of the
carburetor are to meter, atomize and distribute the fuel.
Fuel-Air Requirements.............................................. 6M-8
Power vs. Economy.................................................. 6M-8
Cold Starts................................................................... 6M-9
Basic Carburetor Systems........................................ 6M-9
Float System................................................................ 6M-10
Idle System.................................................................. 6M-10
Main Metering System............................................ 6M-11
Power System.............................................................. 6M-12
Pump System.............................................................. 6M-12
Choke System............................................................. 6M-12
E N G I N E FUEL 6 M - 3
LIGHT DUTY TRUCK SERVICE MANUAL