2008 Suzuki LT-A400/F, LT-F400/F ATV Service Manual, Page 171Get this manual
Fuel System:
Diaphragm and Piston Operation The carburetor is variable-venturi type, whose venturi cross sectional area is increased or decreased automatically by the piston valve (1)The piston valve moves according to the negative pressure present on the downstream side of the venturi "A"Negative pressure "B" is admitted into the diaphragm chamber (2) through an orifice "C" provided in the piston valve (1)Rising negative pressure overcomes the spring (3) force, causing the piston valve (1) to rise into the diaphragm chamber and prevent the air velocity from increasingThus, air velocity in the venturi passage is kept relatively constant for improved fuel atomization and the precise airfuel mixture2 [A] [B]