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
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
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
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
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.
The carburetor acceleration system, like the off idle
system, provides extra fuel when changing from the idle