Buell S1 Lightning 1996 Manual page 180

Mid-Range Slide Position
See Figure 4-6. As the throttle plate is opened, air flow
increases through the carburetor; this causes air pressure to
decrease in the carburetor venturi (near the needle jet) and in
the chamber above the diaphragm (which is vented to the
venturi through a vacuum port and passage in the vacuum
piston).
The chamber beneath the diaphragm is vented to higher atmo-
spheric pressure by a passage to the carburetor inlet. The
higher air pressure at the underside of the diaphragm over-
comes spring pressure and moves the vacuum piston upward
in proportion to the pressure difference between the chambers.
Throttle
plate
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The tapered needle moves upward with the vacuum piston,
thereby opening the needle jet. With the needle jet open, the
main bleed tube is exposed to the lower pressure of the car-
buretor venturi. This causes fuel in the float bowl (at atmo-
spheric pressure) to flow through the main jet and into the
main bleed tube. Air from the main air jet (at atmospheric
pressure) flows through the main bleed tube openings and
mixes with the incoming fuel. The air/fuel mixture is then
delivered through the needle jet into the main air stream of
the venturi.
Main bleed
tube
Figure 4-6. Mid-Range Slide Position
Diaphragm
Spring
Vacuum piston
Vacuum port
Needle
Needle jet
Main air jet
AIR
Main jet
FUEL
AIR/FUEL
4-9