Zenith Model #33 Carburetor; Operation And Service; Fuel Supply System; Idle System - Ford VSG-411 Service Manual

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Fuel System 03-7
ZENITH MODEL #33 CARBURETOR
Operation and Service Idle System
The Zenith model #33 carburetor is a singe 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 I-1 /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.
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.
SECONDARY IDLE
DISCHARGE HOLE
IDLE AIR BLEED
JET AND
-UP TUBE
ADJUSTI 1
b7-y NEEDLE
IDLE SYSTEM
The discharge of the idle fuel into the air stream at curb
idle is controlled by the idle adjusting needle in the primary
idle discharge hole. Turning the needle in (clockwise)
restricts the primary discharge hole to provide a leaner
mixture. Turning the needle out (counterclockwise) allows
a richer mixture.