With the exception of electric-start models, most walk-behind lawnmowers with gasoline-powered engines are started by pulling on a starter cord. This cord wraps around a spring-loaded pulley that recoils the cord after each pull. After years of recoiling the pulley after engine starts, the recoil spring can be weakened and break. If your recoil spring needs replacement, the repair takes about 15 minutes with some common hand tools.
Disconnect the sparkplug wire from the sparkplug by pulling on the black rubber boot covering the tip of the sparkplug.
Loosen and remove the bolts that secure the starter shroud to the top of the engine, using the socket set. Lift off the shroud and place it upside-down on a work table.
Bend the three tabs that hold the pulley into the starter shroud upward until they are pointed vertically. Carefully lift the starter pulley out of the starter shroud, as there could be some tension in the recoil spring underneath the pulley. Cut the starter cord with a utility knife, and remove the pulley from the starter shroud.
Grab onto the knot in the knot hole near the centre of the pulley, and pull out the section of starter pull cord. Discard any sections of the old starter spring from the pulley and/or the starter shroud.
Slip the new tightly-wound recoil spring into the top of the starter housing. Hook the end of the spring into the spring catch in the housing. Gingerly remove the four bands around the spring to loosen the spring in the housing. Hold your hand over the spring in the event that it tries to spring out of the housing as you remove the bands.
Nestle the pulley into place over the spring and rotate it counterclockwise to engage the spring with the pulley. Hold the pulley in place, and bend the three tabs back down to hold the pulley in position.
Rotate the pulley counterclockwise as far as it will go. When it stops turning, clamp the pulley in place with a pair of wide-jaw locking pliers.
Insert the end of a new pull cord (with the handle attached to the opposite end) into the hole in the starter shroud, guiding the cord into the pulley toward the knot hole. When the cord appears in the knot hole, pull out about a foot of cord through the knot hole with a pair of needle-nose pliers.
Tie a knot in the end of the cord, then singe the remaining cord with a match to prevent fraying.
Pull on the cord from outside the starter shroud to seat the knot firmly in the knot hole. Release the wide-jaw locking pliers and allow the pulley to recoil the cord.
Replace the starter shroud onto the top of the engine, and secure it by tightening the bolts you removed. Reconnect the sparkplug wire to the sparkplug.