At small throttle openings the vacuum created by the main venturi is not sufficient to cause fuel to flow from the nozzles. Therefore, an additional system has been provided to furnish the proper mixture ratios required throughout the low speed range.
An adjustable idle system is used in the primary side of the carburetor which supplies the fuel required for normal curb idle, as well as that required for operation in the off idle and low speed range. The idle fuel passes from the float bowl through the main metering ejts into the main well area. The fuel then travels up the idle tube, past an air bleed, through the idle restriction, and past another air bleed. The mixture then travels down through a passage in the bowl, past the lower idle air bleed, (where used) and then past the off idle ports where additional air is bled into the mixture. The mixture is then discharged into the throttle bores from the idle needle holes.
Some applications have a fixed idle system on the secondary side. The quantity of air/fuel mixture is controlled by the size of the discharge hole located below the throttle valves on the secondary side.
As the throttle valves are opened from the curb idle position, the air entering the off-idle discharge holes gradually diminishes. When these holes become exposed to manifold vacuum, they then become fuel discharge holes.
Further opening of the throttle valves increases the air velocity through the carburetor sufficiently to cause the air to strike the end of the extended lower idle air bleeds, thus creating a low pressure within the bleed tube. As a result, fuel begins to discharge from the lower idle air bleed tubes and continues to do so through the part throttle and wide open throttle ranges supplementing the main discharge nozzle delivery.
IDLE AIR BY-PASS SYSTEM
Some model 4GC carburetors use an idle air by-pass system. The purpose of this system is to allow the primary throttle valves to be completely closed during curb idle operation. This design prevents carbon and gum formations which may form around the throttle vavles from disrupting engine idle speed.
The fuel flow in this system is basically the same as in the standard idle system described previously. However, the idle air which normally passes by the slightly open throttle valve is passed around the throttle vavles through an idle air by-pass channel.
In this system, idle air is taken from the carburetor bore above the throttle valves, by-passes around the closed throttle valves, through an air channel and enters the carburetor bore just below the throttle valves. The amount of idle air which is supplied to the engine is regulated by an idle air adjustment screw located in the idle air passage. The adjustment screw is located on most models at the left rear of the carburetor as mounted on the engine. Turning the screw inward (clockwise) lowers the engine idle speed and turning it outward (counter-clockwise) increases the engine speed.
In order to obtain sufficient idle air for stable idle speed adjustment, a supplementary or fixed idle air bleed is used in addition to the adjustable idle air screw. The fixed air bleed can either be a calibrated hole drilled through each primary throttle valve or a calibrated fixed idle air channel which leads from above the primary throttle valve to below the valve.
The type used is dependent upon which is acceptable to the particular engine design.
When adjusting engine idle speed with the idle air by-pass system, use the following procedure:
Start and warm up the engine thoroughly. Make sure choke is completely open and both throttle valves are completely closed
Connect a tachometer to the engine and turn the idle air by-pass screw in or out until the specified idle RPM is reached
Adjust the two idle mixture screws to obtain the highest RPM and a smooth idle