The starter rope on a Stihl FS 55 trimmer connects the engine and the flywheel, allowing you to fire the spark plug and make cold and warm starts. According to the Stihl FS 55 trimmer service manual, "if the action of the starter rope becomes very stiff and the rope rewinds very slowly or not completely, it can be assumed that the starter mechanism is in order but plugged with dirt." In this case, or if the starter rope breaks, you will need to install a replacement.
Unscrew the three screws holding the rewind starter rope in place. On the FS 55, the rewind starter rope is at the rear of the engine. Remove the cover that holds the rewind starter in place. Turn the cover over and wipe the starter assembly clean with the rag.
Disengage the recoil spring mechanism from the pulley and release the tension on the starter rope. Pull the starter rope out 6 inches. Hold the pulley mechanism and keep it from spinning.
Pull the starter rope into the notch on the pulley and make a loop with the rope. Continue holding the pulley while turning the loop clockwise until the spring disengages.
Unwind the starter rope from the pulley and cut it off with the scissors at the point where it is connected to the pulley. Cut off the rope where it is tied to the starter handle. Discard the old rope--but keep the handle for the new rope.
Thread the new rope into the hole on the pulley and through the hole on the starter housing. Tie an overhand knot in the end of the rope above the pulley hole to fasten it to the pulley. Thread the starter rope through the hole on the starter handle. Tie an overhand knot at the end. Tug on the rope a few times to make sure it is secure.
Wind the starter rope around the pulley. Engage the recoil spring by pulling 6 inches of rope up and pushing it into the slot on the pulley. Make a loop in the rope and wind the pulley counterclockwise seven times.
Hold the pulley while you unravel and straighten out the tightly wound loop portion of the starter rope. Continue holding the starter handle to keep the tension tight. Let go of the pulley and slowly let go of the starter handle. The remaining rope should rewind; if it doesn't, repeat the tensioning process.
Set the starter rope in place over the starter housing area. Screw down the three screws to secure the rope and cover in place.