The mechanism that starts your lawn mower is pretty simple. The pull start cord attaches to a pulley. When that pulley is spun by a pulled starter cord, it fires the engine into action. But each time you yank on the pull start cord to spin the pulley, the rope must withstand considerable force and friction. And inevitably the cord succumbs and breaks. Once you repair the cord, it will likely break again. So it's a good idea to learn how to repair your lawn mower's pull start now.
Expose the starter pulley. Depending on the make and model of the lawn mower, you will likely have to open the lawn mower's casing and any housing covering the starter pulley. For most models, you'll remove a series of bolts.
Remove the broken pull start cord from the starter pulley. Depending on the model of your lawn mower, you may have to remove the starter pulley. For most models, this can be done by removing the center bolt. If the pull start cord is still wrapped around the starter pulley, note the number of times it is wrapped around the pulley before removing it.
Cut a new pull start cord that is approximately the same length as the pull start cord you removed. Wrap a small piece of electrical tape around each end of the pull start cord to make it easier to thread.
Thread one end of the pull start cord through the small hole located on the starter pulley where the old hole was attached. Tie a knot in the end of the pulley cord to secure it. You may want to detach the electrical tape first to make knotting easier.
Wrap the starter cord around the starter pulley. If you noted the number of times it was previously wrapped, wrap the new cord around the same number of times. If not, wrap the cord around twice. Then reattach the starter pulley (if removed) and pull the starter cord to see if you have wrapped the right amount of cord to turn the pulley with one good tug. If not, adjust.
Thread the free end of the starter cord through the hole in the starter pulley's housing cover and reattach it. Then reattach the lawn mower cover.
Attach the free end of the cord to the pull start's handle. Before you do this, thread the pull start's cord through any loops that attach it to the handle of the lawn mower. Then, thread the pull start cord through the bottom of the pull handle. Secure the pull start cord by threading it through the handle and out of the top, then tying it with a knot. Or duplicate whatever method orignally attached the cord to the handle.