Buell S1 Lightning 1996 Manual page 179

Idle- and Low-Speed Circuit
See Figure 4-5. At idle (with the throttle plate closed and the
main air stream obstructed), engine idle speed is maintained
by fuel metered through the slow jet. Air from the slow air jet
mixes with the fuel and is delivered to the idle port at the low
pressure side of the throttle plate.
At low-speed (with the throttle plate slightly open), the transfer
ports are exposed to the low pressure side of the throttle plate,
and additional fuel is directed to the barrel of the carburetor.
Throttle plate
Transfer ports
Idle port
Idle mixture
screw
4-8
Slow jet
Figure 4-5. Idle- and Low-Speed Circuit
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During the transition period from idle speed to mid-range, the
idle and transfer ports also supply some fuel to the carburetor
barrel; this allows for a smoother transition.
The venturi opening is reduced by the low position of the vac-
uum piston. This enables initial air stream velocities to be
higher than normally attainable with fixed-venturi carburetors.
The higher air stream velocities provide improved atomization
of fuel necessary for good acceleration and driveability.
Spring
Diaphragm
Vacuum piston
Vacuum piston
Vacuum port
Slow air jet
AIR
FUEL
AIR/FUEL