Float System - Chevrolet Light Duty Truck 1973 Service Manual

6 M - 1 0 ENGINE FUEL
it does so under such varied conditions that it is
necessary to have several systems to alter its functions so
that it can adjust to various situations. Most carburetors
contain the following six basic systems:

• Float System

• Idle System
• Main Metering System
• Power System
• Pump System
• Choke System
Float System
(Fig. 14)
Fuel in the carburetor float bowl must be maintained at
a specified level for correct fuel metering under all
driving conditions. The float system accomplishes this by
using a float pontoon and attached leverage arm which
exerts force against a needle valve, shutting off fuel flow
when the specified level is reached in the carburetor
bowl. Fuel enters the inlet and fills the carburetor bowl
through the valve orifice (needle seat).
As the level in the bowl rises, the buoyant action of the
float seats the needle valve. When fuel is being used
from the bowl, the float drops sufficiently to allow the
needle to unseat and fuel to enter past the needle to
maintain,the specified level in the float bowl.
The liquid level controlled by the float setting is an
im portant part of the calibration of the carburetor. The
effects of a low liquid level causes poor performance in
the main metering system and could cause loss of power.
High liquid level can result in premature main metering
delivery and fuel spillage during normal car maneuver­
ing, each of which causes excessive fuel consumption and
an over-rich condition. The fuel level is controlled by a
float and adjusted by bending a tab on the float arm.
Accuracy of this adjustment may be measured by
checking the physical relationship of the float to a
particular area on the fuel bowl.
The float system is perhaps one of the most important
systems in the carburetor, as the correct operation of all
other systems depends on the proper level of fuel in the
float bowl.
Idle System
(Fig. 15)
During engine idle and low speed operation, air flow
through the carburetor is very slight due to the throttle
blade being nearly closed and is insufficient to meter fuel
properly from the main discharge nozzle. To offset this
condition, the idle system is used to provide the proper
mixture ratios required during engine idle and low speed
operation. The idle system consists of an idle tube, idle
passages, idle air bleeds, off-idle discharge ports, idle
mixture adjusting needle and the idle mixture needle
discharge port.
In the idle speed position, the throttle valve is slightly
open, allowing a small amount of air to pass between the
wall of the carburetor bore and the edge of the throttle
valve. Since there is not enough air flow for venturi
action, the fuel is made to flow by the application of
manifold vacuum directly through the idle system to the
fuel in the carburetor bowl.
The low pressure below the throttle valve (manifold
vacuum) will cause the fuel to flow through the idle tube,
into the idle passage, where it is mixed with air from the
air bleed. This is the first stage of atomizing the fuel.
The mixture continues down the passage, past the off-idle
ports. At this point these ports act as air bleeds to further
break up the mixture. The mixture flows past the mixture
screw into the carburetor bore and into the engine intake
manifold. The mixture screw controls the idle mixture
and is turned in to lean the mixture and out to richen it.
Mixture screws are now locked after flow test at the
manufacturing plant by means of limiter caps which
attach over the mixture screws. These caps must not be
tampered with in the field due to the effect on exhaust
emissions.
As the throttle valve is opened, additional air flows
through the carburetor. Since air flow is still insufficient
to draw fuel from the main nozzle, the increased air flow
results in excessively lean mixtures. To compensate for
this problem off-idle ports are added to the carburetor
(inset Fig. 15) just above the closed throttle position.
When the throttle is opened during slow speed or off-idle
operation (Fig. 15), the off-idle ports are exposed to
manifold vacuum and begin discharging extra fuel
mixture for off-idle requirements. Thus, the off-idle ports
have a dual purpose. At idle they act as air bleeds but
LIGHT DUTY TRUCK SERVICE MANUAL