Fuel Supply System; Idle System - Ford LRG-423 Service Manual

03-24
Auxiliary Systems
03-24
ZENITH MODEL #33 CARBURETOR OPERATION AND SERVICE
The Zenith model #33 carburetor is a single downdraft
unit of two-piece construction, consisting of a cast
aluminum air intake and throttle body with a fuel bowl.
The single venturi is removable for easy adaptability to
meet different size engine requirements.
The mounting flange holes are elongated to allow it
to fit either the SAE
1"
or 1 l/4" flange. A vacuum-
operated accelerator pump is used.
Fuel Supply System
The fuel supply system consists of the threaded fuel
inlet connection and fuel valve seat, fuel valve, float
and fuel chamber. The fuel supply line is connected to
the threaded inlet. Gasoline passes through the
connection, through the fuel valve seat and past the
fuel valve and into the fuel chamber. The level of the
fuel in the fuel chamber is regulated by the float
through its control of the fuel valve. The fuel valve
does not open and close alternately. It assumes a
more or less fixed-opening position as regulated by
the float, sufficient to maintain a proper level in the fuel
chamber equal to the fuel demand of the engine
according to its speed and load.
Idle System
The idle system supplies fuel to run the engine at curb
idle and also slow speeds until sufficient velocity is
built up in the main venturi to operate the main system.
This system consists of the idle discharge holes, the
idle adjusting needle, the passage or channel between
the idle jet and the discharge holes, the idle air bleed,
and the idle jet and pickup tubes. At idling speeds,
manifold vacuum is transmitted through the primary
idle discharge hole to the idle jet through a passage
running through the throttle and air intake bodies. Fuel
from the fuel bowl flows through the main jet into the
fuel well where it is picked up by the pickup tube to be
metered by the idle jet. As the fuel leaves the idle jet it
is mixed with air from the idle air bleed, which is
located just above the jet in the air intake, therefore a
fuel and air mixture is provided just before the
passage to the idle discharge holes. This air bleed
calibration is to provide better control of the fuel
delivery and to prevent the fuel in the bowl from being
siphoned into the intake manifold through the idle
system when the engine is shut off.