Main Metering System; Power Fuel System; Accelerator Pump System - Ford LRG-423 Service Manual

03-35
Auxiliary Systems
03-35
HOLLEY ONE-BARREL CARBURETOR MODEL 1930 (Continued)
Inr c
IDLE AIR
IDLE WELL
' MIXTURE
SCREW
PPD0017
Main Metering System
In the main metering system, the engine approaches
mid range speed. The air flowing through the venturi
increases, thus creating a low pressure area in the
carburetor venturi. Fuel in the fuel bowl is at nearly
atmospheric pressure. The low pressure, created by
8 the venturi and magnified by the duel booster venturi,
forces the fuel in the fuel bowl to flow through the
discharge nozzle.
Fuel flows through the main jet into the main well; air
enters through the main well air bleed and into the
main well through holes in the main well tube.
Because the air/fuel mixture is lighter than raw fuel, it
responds faster to venturi vacuum changes and is
vaporized more readily when discharged into the
venturi.
I
MAIN WELL
HIGH SPEED
TUBE
AIR BLEED
DISCHARGE
I MAIN
\
METERING JET
MAIN WELL
PPDOOI 8
The main discharge nozzle passage is part of the dual
booster venturi, which is an integral part of the main
body casting.
The main metering system is calibrated to deliver the
proper mixture for best overall economy. When
additional power is required, a vacuum-operated
power system enriches the air/fuel mixture.
Power Fuel System
The power fuel system consists of a power valve
installed near the center of the carburetor main body
and a vacuum piston installed in the fuel bowl cover. A
vacuum passage leads from the top of the piston down
to the manifold flange.
When manifold vacuum is high, the vacuum piston is
raised to the top of its cylinder and the spring in the
piston is compressed.
FLOAT
\
VACUUM
/ PISTON
MAIN WELL MAIN JET
PPDOOI 9
When manifold vacuum drops to a predetermined
level, the spring overcomes the vacuum and pushes
the piston stem down. The piston stem, in turn, pushes
the power valve down, opening the power valve and
permitting fuel to flow through the power valve,
through the channel restriction and into the main well
located near the power valve.
Accelerator Pump System
Upon sudden opening of the throttle plates, airflow
through the carburetor increases almost immediately;
however, there is a brief time interval or lag before the
fuel can overcome its inertia and attain required flow to
maintain the desired air/fuel ratio.
The piston-type accelerating pump system
mechanically supplies the fuel necessary to overcome
this deficiency for a short period of time.
Fuel enters the pump cylinder from the fuel bowl
through the pump cup stem clearance hole when the
pump is lifted to a refill position. The fuel level is above
the normal position of the pump piston. This is known
as a wet pump system.