The primer bulb on your Craftsman lawn mower engine is attached to the carburetor. This small bulb, when pressed, forces a small amount of fuel into the carburetor. Without this first squirt, the mower would not start. When you pull on your mower's starter rope, the gas you pumped into the carb with the primer gives the mower engine juice to start. Then vacuum power pulls more gas into the carb as the mower runs. A Craftsman mower primer bulb is a thumb-size soft plastic bubble that can crack over time. At some point, the primer bulb will need to be replaced. The hardware you'll need for a Craftsman mower primer is available at hardware stores and cost only a few dollars.
Remove the Craftsman mower's air filter by turning the screw holding it in place. Take off the cover, the filter and the lower cover. This should expose the primer bulb.
Pry off the plastic ring around the primer bulb using a flat-head screwdriver.
Turn the plastic clip that holds the primer bulb in place, using your screwdriver, until it comes loose. Remove both clips holding the primer bulb in place.
Place the new primer bulb into the hole and put the new clips around it to hold the bulb in place. Attach the new plastic cover around the primer bulb.
Replace the air filter cover and attempt to start the engine to test the bulb. Ensure there are no leaks coming from the bulb.