Buell S1 Lightning 1996 Manual page 182

Accelerator Pump System
See Figure 4-8. The accelerator pump system uses sudden
throttle openings (rapid acceleration) to quickly inject fuel into
the carburetor venturi; the extra fuel provides for smooth
acceleration. This fuel also assists engine operation during
cold engine warm-up when the enrichener is turned off pre-
maturely.
Rapid throttle action, during the first third of throttle travel,
causes the accelerator pump rod to depress the accelerator
Venturi
Pump
nozzle
Check
valve
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Pump lever
Diaphragm
Figure 4-8. Accelerator Pump System
pump diaphragm. This forces fuel in the pump to flow through
a fuel passage (which has a "one-way" check valve), through
the pump nozzle, and then into the venturi. When the throttle
closes, the pump rod lifts up and away from the pump
diaphragm; a spring below the diaphragm pushes the
diaphragm upward, thereby causing the lower pump cavity to
refill with fuel from the float bowl. The check valve prevents
backflow of fuel from the pump nozzle/fuel passage during
this refilling phase.
Pump
rod
Spring
Throttle
shaft
Throttle
rod
Spring
FUEL
4-11