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Ah yes, the Tillotson model H diaphragm carburetor designed for chainsaws. I barely remember the purge valve. Actually not at all till you mentioned it. The deal is, the arm that opens the inlet needle valve is held closed by that little spring. When the engine is running, gas is sucked from the diaphragm chamber, causing a vacuum which moves the diaphragm towards the lever, over-coming the spring, and more gas can enter the diaphragm chamber. Not much different than a float valve carb, if you think about it. In chainsaw use, with the gas tank above or at least close, yanking the pull rope a dozen times eventually creates enough suction to open the diaphragm valve and hopefully it starts and continues to run. Different story when they put the carburetor a foot or more above the gas tank. Hense the purge valve to get the gas at least close to the carb and maybe start with a dozen pulls.
As for the details of the purge system, duh, I forget.