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I HAVE A WLA-1 carb on my 500t blower , my question is can someone tell me what the (piston assy is, number 12318539130) and what it does, im thinking its some kind of power valve, its brass has a oring, and spring

Without looking it up sounds like a mixture screw

That piston assembly is used to apply mechanical pressure to the diaphragm of the internal fuel pump.