The PUMP CHECK BALL only allows fuel to
flow into the pump reservoir. It stops fuel from flowing
back into the fuel bowl when the pump is actuated.
The PUMP CHECK WEIGHT prevents fuel
from being pulled into the air horn by venturi vacuum.
Its weight seals the passage to the pump nozzle and
prevents fuel siphoning.
The PUMP NOZZLE, also known as the pump
jet, has a fixed opening that helps control fuel flow out
of the pump. It also guides the fuel stream into the
center of the air horn.
The basic operation of the acceleration system is as
The pump piston or diaphragm is pushed down
in the pump chamber, as the throttle plate is opened,
forcing fuel through the outlet passage.
At the same moment, the pump check ball will
seat, keeping fuel from being pumped back into the float
The pump check weight will be forced off its
seat, allowing fuel to pass to the pump discharge nozzle,
and then discharged into the carburetor.
The pump piston or diaphragm is raised in the
pumping chamber when the throttle plate is closed,
causing the pump check weight to seat blocking the
At the same time, the pump check ball is pulled
off its seat and fuel is pulled into the pump chamber
from the float bowl.
The pump chamber is filled with fuel and ready
for discharge whenever the throttle plate is opened.
The linkage between the accelerator pump and the
throttle cannot be solid. If it were, the pump would act
as a damper, not allowing the throttle to be opened and
closed readily. The linkage activates the pump through
a slotted shaft When the throttle is closed, the pump is
held by its linkage. When the throttle is open, the pump
is activated by being pushed down by a spring that is
called a duration spring (fig. 4-24). The tension of the
duration spring controls the length of time that the
stream of fuel lasts. The spring is calibrated to specific
applications. Too much spring pressure will cause fuel
to be discharged too quickly, resulting in reduced fuel
economy. Too little spring pressure will result in the
fuel being discharged too slowly, causing engine
The, high-speed system, also called the main
metering system, supplies the engine air-fuel mixture at
normal cruising speeds. This system begins to function
when the throttle plate is opened wide enough for the
venturi action. Air flow through the carburetor must be
relatively high for venturi vacuum to draw fuel out of
the main discharge tube. The high-speed system
Figure 4-24.Duration spring.