3-12
Fuel System
3-12
HOLLEY MODEL 1904
IDLE FUEL SYSTEM
During Idle, fuel passes through the main jet into
ONE-BARREL CARBURETOR
the bottom of the main well (Figure 26). High manifold
DESCRIPTION
vacuum acting through the idle passages draws fuel
The Holley model 1904 single-barrel carburetor
from the main well through a short horizontal passage
(Figure 24) is used on the 300 CID engines. It consists
into the idle well.
of two main assemblies. the main body and the throttle
The fuel is metered through a calibrated
body.
restriction, at the top of the idle well, on its way into the
The main body contains the float, fuel-inlet valve,
Idle channel. Air Is introduced through an idle air bleed at
fuel bowl, the carburetor air inlet, the main and booster
the top of the Idle channel. The air bleed also acts as a
venturi, the choke plate, the main well body. the power
vent to prevent siphoning at high speeds or when the
fuel assembly, and the accelerating pump assembly.
engine is stopped. The fuel-air mixture travels down the
The main well body contains the majority of the fuel
idle channel past two Idle transfer holes in the throttle
passages and the fuel metering parts.
body and is discharged through the idle discharge hole
The throttle body houses the throttle plate, the
below the closed throttle plate. As the throttle plate is
Idle discharge ports, the idle speed screw. and the Idle
moved past the two transfer holes, during off idle, each
fuel mixture adjustment needle.
hole begins to discharge fuel as it is exposed to manifold
OPERATION
vacuum. The transfer holes act as additional air bleeds
The carburetor has four fuel metering systems to
at idle.
provide the correct fuel-air mixture for all phases of
engine operation. The four systems are: the idle fuel
Fuel discharge at idle is controlled by an Idle
system, the main fuel system, the accelerating system.
adjusting needle which seats in the discharge hole.
and the power fuel system. In addition, a fuel inlet
system regulates the fuel supply to the various systems,
and a manual choke provides an enriched mixture to aid
In starting and running a cold engine.
FUEL INLET SYSTEM
Fuel under pressure from the fuel pump enters
the float chamber through the fuel inlet needle valve and
seat assembly (Figure 25).
Movement of the needle valve in relation to the
seat is controlled by the float and lever assembly which
rises and falls with the fuel level. As the fuel level drops.
the float lowers, opening the needle valve to admit fuel.
When the fuel in the float chamber reaches a pre-set
level. the float moves the needle valve to a position
where it restricts the
FIG. 25. Fuel Inlet System
FIG. 24. Holley Single-Barrel Carburetor
flow of fuel into the float bowl. Changes In the fuel level
cause a corresponding movement of the float which
opens or closes the needle valve to maintain the pre-set
fuel level. This level must be maintained because the
carburetor is calibrated to deliver the proper mixture only
when the fuel Is at this level.
A spring and pin are located in the hollow needle
FIG. 26. Idle Fuel System
valve to cushion the valve against vibrations. A clip, to
assure reaction of the valve to any float movement, is
attached to the valve and float.
