Cub Cadet 751-10639A  Primer Bulb

The Primer Bulb is a small round pliable plastic bubble or button located on the carburetor that when pressed draws fuel through the fuel line into the carb. Over time, exposure to chemicals in the fuel, grit from the fuel tank, and heat from the sun can harden the plastic primer bulb causing it to get brittle and crack. Home users often replace the primer bulb during their annual tune-up. Replacing the primer bulb is usually very simple and requires little more than a screwdriver.