2008 Suzuki LT-A400/F, LT-F400/F ATV Service Manual, Page 170Get this manual
1G-3 Fuel System: Fuel Valve When the engine is not operating, the fuel valve (1) is kept closed by the tension of the spring (2), which closes the fuel passageway and stops the flow of fuel to the carburetorWhen the engine has started, negative pressure (vacuum) "A" is generated in the combustion chamber and reaches the diaphragm through passage in the carburetor's main bore and the vacuum hoseThis negative pressure (vacuum) "A" is higher than the spring pressure which causes the diaphragm to force open the fuel valve (1) and allow fuel "B" to flow to the carburetor assembly [A]