The pull cord on a Ryobi trimmer is connected to the starter, which fires the spark at the proper time, thus starting the engine. This cord can become damaged or ripped off the pulley inside the starter cover. There is also a recoil spring inside the pulley that, if not set properly, will not allow the rope to give the engine full compression, making starting the trimmer very difficult. If any of these things happen, you must replace the cord.
Locate the starter cover, which on most Ryobi's is at the rear of the trimmer's engine and contains the pull cord, pulley and recoil spring on the reverse side. Use the screwdriver to remove the four screws holding the cover in place. Remove the cover.
Flip the cover over to work on the pull cord. Use the screwdriver to pop open the center locking tab, which keeps the cord connected to the pulley and must be opened for you to replace the cord.
Pull out the cord from the pulley. Use the screwdriver to wedge open the plastic starter handle on the other end of the cord. Remove the metal insert and cord from the handle.
Untie the cord from the metal insert. Pull the cord all the way through the hole on the open locking tab. Throw the old cord away.
Insert the replacement cord through the same hole on the pulley's center. Pull it all the way through until its other end is inside the pulley. Push the locking tab down to secure the cord to the pull. Tug on the cord a few times to make sure it will hold under pressure.
Thread the cord through the hole on the starter cover, the metal insert and the starter handle. Tie a simple, overhand knot around the metal insert using only the end of the cord. Don't tighten it around the metal insert yet.
Push the cord and metal insert back into the starter handle. Close the starter handle, and pull down on the cord to tighten it in place. Tug on the cord a few times to make sure it will hold under pressure.
Wind the cord around the pulley. While holding the starter cover, pull out on the cord so the pulley makes two full rotations. The cord should set the tension on the recoil spring and retract back into the pulley.
Repeat pulling the cord out two full pulley rotations until the tension is properly set. If tension can't be set, make sure the cord is properly attached to the pulley. If it still won't set, replace the recoil spring.
Put the starter cover back in place over the engine. Use the screwdriver to reattach and tighten the screws in place. Test the cord by pulling it out of the pulley. The cord should retract fully.