Husqvarna 537275001  Purge Bulb

The Purge Bulb is a small round pliable plastic bubble or button located on the carburetor that removes air from the fuel line. An engine with a purge bulb also often has a choke plate. Over time, exposure to chemicals in the fuel, grit from the fuel tank, and heat from the sun can harden the plastic Purge Bulb causing it to get brittle and crack. Home users often replace the purge bulb during their annual tune-up. Replacing the purge bulb usually requires little more than a screwdriver.