Idle System

The carburetor idle system (fig. 4-21) provides the air-fuel mixture at speeds below approximately 800 rpm or 20 mph When the engine is idling, the throttle is almost closed Air flow through the air horn is restricted to produce enough vacuum in the venturi. Since venturi vacuum is too low to pull fuel from the main discharge tube, the high intake manifold vacuum BELOW the throttle plate and the idle circuit are used to feed fuel into the air horn.

The fundamental parts of the carburetor idle system include a section of the main discharge tube, a low- speed jet, an idle air bleed, a bypass, a idle passage, an economizer, an idle screw port, and an idle mixture screw.

The LOW-SPEED JET is a restriction in the idle passage that limits maximum fuel flow in the idle system. It is placed in the fuel passage before the idle air bleed and economizer.

The IDLE AIR BLEED works with the economizer and bypass to add air bubbles in the fuel flowing to the idle port. The air bubbles help break up or atomize the fuel. This makes the air-fuel mixture burn more efficiently once it is in the engine.

The IDLE PASSAGE carries the air-fuel slurry (mixture of liquid and air bubbles) to the idle screw port.

The IDLE SCREW PORT is an opening into the air horn below the throttle valve.

The IDLE MIXTURE SCREW allows adjustment of the size of the opening in the idle screw port. Turning the screw IN reduces the size of the idle port and the amount of fuel entering the horn. Turning the screw OUT increases the size of the idle port and enriches the fuel mixture at idle.

Figure 4-21. - Idle system.

Most modern carburetors have sealed idle mixture screws that are NOT normally adjusted. The seal prevents tampering with the factory settings of the idle mixture. Sometimes a plastic limiter cap is pressed over the idle mixture screws. They restrict how far the screws can be adjusted toward the rich or lean settings. Correcting idle screw adjustment on modern carburetors is critical to proper exhaust emission.

The basic operation of the idle system is as follows:

At idle, fuel flows out of the fuel bowl, through the main discharge tube, and into the low-speed jet. The low-speed jet restricts maximum fuel flow.

At the bypass, outside air is pulled into the idle system. This partially atomizes the fuel into slurry. As the air and fuel bubbles pass through the economizer, the air bubbles are reduced in size to further improve mixing.

The fuel and air slurry then enters the idle screw port. The setting of the idle screw controls how much fuel enters the air horn at idle.

With the throttle plate closed, high intake manifold pressure pulls fuel out of the idle system.

Off Idle System

The off idle, also known as the part throttle, feeds more fuel into the air horn when the throttle plate is partially open. It is an extension of the idle system. It functions above approximately 800 rpm or 20 mph. Without the off idle system, the fuel mixture would become too lean slightly above idle. The idle system alone is not capable of supplying enough fuel to the air stream passing through the carburetor. The off idle system helps supply fuel during the change from idle to high speed.

Basic off idle system operation is as follows:

The driver presses down on the accelerator and cracks open the throttle plate. As the throttle plate swings open, the off idle ports are exposed to intake manifold vacuum.

Vacuum then begins to pull fuel out of the idle screw and the off idle port. This action provides enough extra fuel to mix with the additional air flowing around the throttle plate.

Acceleration System

The carburetor acceleration system, like the off idle system, provides extra fuel when changing from the idle 4-19