buell s1 lightning '97 Manuel

 

See 

 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.

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.