There are 2 type of valves used,on the tank, one is just the normal petcock with an on/off lever. The other is a vacume operated pump. If that is what you have then you will pull the line feeding the carb then turn engine over. It should spit fuel from the line as engine turns over. If not then pump is defective. If fuel runs from line when engine is sitting still pump is bad as well.Everytime the piston moves down vacume sucks the diaphram down and allows fuel to enter pump chamber, as the vacume subsides a return spring will push the diaphram up forcing fuel through fuel line. Also there is a screen in the tank which will sometimes get obstructed from sediment. You can also pull the vacume line and apply suction to it and fuel should flow.
TOM
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