A dual line trimmer contains two separate plastic lines inside the bump head. These lines connect to an inner spool and spring. When you push the bump head down during operation, the spring ejects new line off the spool. Filling the spool properly will allow you to advance the line as you go, and you won't have to stop and unclog a jam inside the spool. You can purchase trimmer line on a bulk roll to save money on prepackaged spools.
Turn the ignition switch off. Set the trimmer on the ground and allow it to cool if the engine is hot.
Place the powerhead on the ground and hoist the shaft into the air. Cradle the shaft in the crook of your arm. Grab the moving head attachment and keep it from spinning.
Push the bump knob's spring down into the head and turn it according to the directional arrow on the cover. Pry the cover off the trimmer head. Pull out the inner spool and the spring.
Cut or remove any remaining bits of trimmer line still attached to the spool. Wipe the spool, spring, cover and housing area with the rag. Check these parts of signs of cracking or breaking. Replace parts immediately if damaged.
Cut off two sections of line from the bulk roll. Use the appropriate length of trimmer line, which can be found in your operator's manual. Common dual line trimmer lengths range from 6 to 25 feet, so you should verify the specific length of line for your model.
Insert one of the lines into the eyelet on the upper half of the spool. Some trimmers may have a notch in the upper flange of the spool. Thread 3/4-inch of line through the eyelet or notch.
Hold the small section of line down on the spool and wind the line around the spool according to the directional arrow on the spool. Make sure the line winds evenly around the spool. Keep the line tight as you roll it up to prevent jams.
Leave 7 inches of line unwound, and insert it into the notch on the upper part of the spool's flange. Repeat the same winding process for the second length of line. Insert the remaining length into the bottom notch on the spool's flange.
Push the spring and inner spool down into the housing area. Thread the two ends of line through the eyelets on the outer wall of the head. Set the bump knob in place over the cover. Push the knob down and rotate in the opposite direction to lock it in place.
Always make sure your cover is in place before using the trimmer.
The spring inside the trimmer head can cause injury if it springs loose while you are removing the inner spool.