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.
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.
Published
by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy
The carburetor idle system (fig.