Things You'll Need
.095 stock trimmer line
The Ryobi two-cycle 790R trimmer is a combination brush cutter and grass trimmer. Replacing the string on the Ryobi 790R is different from most string trimmers.Instead of just having one end of string, the 790R uses one string with two ends. When replacing the string, you keep the two pieces separate from each other to avoid snags in the string when trimming your lawn. Ryobi recommends using .095 trimmer line as a replacement line for the 790R trimmer. Replacement trimmer line is available at most hardware stores and home improvement centers.
Unplug the spark plug wire from the spark plug with your fingers. Hold the outer spool in one hand, and turn the bump knob counterclockwise with your other hand. Remove the bump knob, and pull the inner string spool out of the outer spool. There is a spring on the top of the inner spool. Pull the spring off and set it aside.
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Remove any leftover string out of the inner string spool. Cut a 40-foot piece of .095 stock trimmer line from a stock spool with a pair of household scissors or utility knife. Hold the string spool with the bottom of the spool facing you. The top of the spool has tabs that hook into the outer housing.
Insert the two ends of the trimmer line into the two small holes near the center of the spool. Pull the ends through the spool so that the resulting loop on top of the spool is as small as possible. Look for the arrow on the bottom of the spool that indicates which direction to wind the line.
Place your index finger between the two pieces of line, and tightly wrap the line around the spool in the direction of the arrow. Your index finger will keep the two pieces of line from overlapping each other as you wind. Continue winding the line until there is only 6 inches of line left on each end.
Hook the remaining 6 inches of line into the string holding slots on the string spool upper tabs. The tabs have small notches that will hold the string ends. Place the ends across from each other in the notches.
Put the spring on the top of the inner spool, and line up the tabs on the inner string spool with the tabs in the outer spool. Insert the inner spool into the outer spool and feed the ends of the trimmer line through the line eyelets in the outer spool.
Push the inner spool inside the outer spool. Thread the bump knob over the bottom of the inner spool to secure the spool. Grab each end of the line and pull to disengage the line from the holding tabs. Push the spark plug wire over the top of the spark plug.