Fuel Metering - Chevrolet Light Duty Truck 1973 Service Manual

6 M - 6 E N G IN E FUEL
DEPRESSION
(LOWER PRESSURE)
Fig. 5 -Air Flow Measurement
understand is that fluid is not pulled by the vacuum but
is rather pushed into the vacuum or low pressure area by
the weight of the air pressing on the surface of the
liquid.
The difference in pressure, or vacuum, is expressed by
measuring the distance between the two heads of water
in the U-tube, or inches of water. Low vacuum is
generally expressed in inches of water but higher
vacuum, such as manifold vacuum, is usually measured
in inches of mercury because mercury is approximately
13 1/2 times heavier than water and thus provides a
P y /..
Fig. 6 -Vacuum Principle
WATER
LOW PRESSURE
ii (VACUUM) DEVELOPED
j!
INSIDE MOUTH
more convenient valve for measuring the larger pressure
differences.

FUEL METERING

As shown in measurement of air flow with the U-tube,
fluids flow when there is a difference in pressure and will
always flow toward the lower pressure. If the fluid level
in the U-tube were higher, the reduced pressure within
the pipe would cause the fluid to flow into the pipe.
Further, if a means were provided to maintain the fluid
level at a desired height in the U-tube, the amount of
fluid flow would be proportional to the pressure
difference caused by the air flow through the pipe.
Carburetors operate on the basic principle of pressure
difference.
A basic carburetor is an adaptation of the U-tube (Fig.
7). The fuel side, open to atmospheric pressure, inside of
air cleaner (internal venting), is enlarged to create a
reservoir and fitted with a float valve to maintain a
constant level of fuel. To control the quantity of fuel
delivered, a jet or metering orifice is screwed into the
base of the main nozzle. A streamlined restriction,
known as a venturi, is added to the air intake side to
create an additional pressure drop for a given rate of air
flow, dictated by upper diameter of the pipe. Finally, a
throttle valve is added to control the volume of air-fuel
mixture admitted.
Applying the basic carburetor to an engine, air flow is
initiated by the pumping action of the pistons and the
intake and exhaust valve action. As the piston moves
downward during the admission or intake stroke, the
inlet valve is open causing air to rush in through the
carburetor and manifold to fill the space left by the
downward piston travel. At engine operating speeds,
especially with multi-cylinder engines, the air flow
through the carburetor is nearly constant. The amount of
air flow, and accordingly the fuel picked up, is controlled
by the opening allowed by the throttle valve.
VENTURI
THROTTLE
Fig. 7 -Basic Carburetor
LIGHT DUTY TRUCK SERVICE MANUAL