2008 Polaris ATV Sportsman 300 400 H.O. Service Manual, Page 99Get this manual
FUEL SYSTEM When the throttle plate is opened and engine speed begins to increase, the pressure in the venturi (and therefore in the chamber above the diaphragm) becomes significantly lower than atmosphericAtmospheric pressure in the chamber below the diaphragm forces the diaphragm upward, raising the slide against spring pressureWhen the pressure above and below the diaphragm are nearly equal, the slide moves downward under spring pressureRaising or lowering the slide increases or decreases the cross sectional area in the venturi, and therefore the air velocity in the venturi is kept relatively constantThis provides improved fuel atomization and optimum fuelair ratio
Starter System (Choke or Enrichment)