Many lawnmower engines use a primer bulb to inject gasoline into the carburettor when the engine is cold. The bulb should feel firm when you press it with your finger, and should pop back into shape when you release it. A primer bulb that does not operate correctly requires repair. You must determine if the clip is faulty or if the bulb itself requires replacement.
Remove the single screw securing the air filter housing to the carburettor, using a flathead screwdriver. The air filter housing is above the primer bulb on top of the carburettor. Pull the housing away from the mower engine.
Clean with a rag around the primer bulb where it makes contact with the crankcase. This removes dirt and grime, so you can see the clip securing the primer bulb to the engine.
Grasp the side of the primer bulb and watch the movement behind the clip. Gently move the bulb right to left with your finger and look for any movement against the crankcase. This indicates the retaining clip is worn and requires replacement.
Look at the bulb and press it in with your fingers. Look for small cracks or holes in the bulb, especially around the edges. Bulbs with cracks or holes require replacement.
Pull the plastic ring off the outer circumference off the primer bulb with your fingers. Find the primer bulb retaining clip on the rear of the primer bulb where it meets the engine. If the retaining clip is loose, attempt to push it into place with a socket and hammer. Place a socket over the ring and tap with the hammer.
Insert a small flathead screwdriver between the inside of the round clip and the side of the bulb housing. There is a notch on each side of the housing. Pry one side of the clip away from the engine, then repeat for the other side. Pull the bulb and the clip away from the mower.
Position the new primer bulb in place and slide the retainer ring over the bulb. Locate a socket that has the same diameter as the retaining ring. Place the socket over the retainer ring and tap it with a hammer to seat the ring past the notches. Push the plastic ring over the primer bulb.
Replacement primer bulbs are available at home-improvement centres. Some have retaining rings in the package. If you bend the current ring, make sure you purchase a replacement for it with the primer bulb.