Buell S1 Lightning Workshop Manual page 211

High-Speed Circuit Slide Position
See Figure 4-7. As the throttle plate is opened, the pressure
difference between the chambers above and below the dia-
phragm increases and the vacuum piston moves further
upward.
Vacuum
port
Throttle
plate
Main bleed tube
4-10
The venturi opening increases and the needle is lifted further
out of the needle jet. The quantity of fuel and the volume of air
are simultaneously increased and metered to the proportions
of engine demand by the variable venturi and needle lift. With
the vacuum piston fully upward, the venturi opening is fully
enlarged and the needle jet opening exposure to the air
stream is at its maximum. Air and fuel supplies are now
available in quantities sufficient to meet maximum engine
demand.
Figure 4-7. High-Speed Circuit
Diaphragm
Spring
Vacuum piston
Needle
Needle jet
Main air jet
AIR
Main jet
FUEL
AIR/FUEL