Navigation bar
  Start Previous page  8 of 25  Next page End Home  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18  

The air flowing through the carburetor responds almost
immediately to any increase in throttle opening. There is,
however, a brief interval before the relatively heavier fuel-air
mixture in the narrow carburetor passages can gain speed and
maintain the desired balance of fuel and air. The accelerating
pump system operates during this interval, supplying fuel until
the other systems can provide the proper mixture.
When the throttle is closed, the pump return spring forces the
pump diaphragm toward the back of the pump chamber,
drawing fuel into the chamber through the pump inlet. The
pump inlet contains a ball check valve which opens to admit
fuel from the float chamber into the pump chamber, and closes
when the pump is operated to prevent a reverse flow of fuel.
When the throttle is opened, the movement is transmitted by
the pump link to the pump operating lever. That lever presses
the pump rod sleeve inward, compressing the pump spring.
The pump spring, in turn, presses on the diaphragm, forcing
the fuel from the pump chamber into the pump discharge
passage. The "overriding" feature
provided by the pump spring assures an even, prolonged
discharge of fuel regardless of how suddenly the throttle
is opened and cushions the action of the pump to prevent
damage to the pump linkage due to those sudden throttle
The fuel, under pressure from the diaphragm, flows
through the pump discharge passage and, forcing the
pump discharge ball check valve and weight up, passes
into the pump discharge nozzle screw. The pump
discharge ball check valve seals the passage when the
pump is not discharging fuel. The hexagonal weight holds
the ball check valve on its seat to prevent a loss of fuel
from the pump chamber due to the siphoning effect of the
airstream at high engine speeds.
Flowing up the hollow pump discharge nozzle screw, the
fuel passes out holes in the head of the screw into the
pump discharge nozzle and is sprayed into the airstream
in the venturi. A slot cut into the pump discharge nozzle
vents the system to prevent the pump discharge ball check
valve and weight from being lifted and fuel drawn from
the pump chamber by the siphoning tendencies of the
airstream at high engine speeds.
Previous page Top Next page