While Stihl makes every effort to factory pre-set the machinery it produces, different environmental conditions may dictate adjusting the unit's idling speed and power output. If your Stihl FS 80 trimmer does not seem to be performing properly, you may need to adjust the carburetor settings. Stihl supplies an adjustment tool with every FS 80 that is designed to help you accomplish this task.
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Restoring Standard Settings
Mount the FS cutting tool, power tool attachment or combination tool (Stihl calls it a CombiTool). Do this with the engine turned off.
Turn the unit so the right side is facing you. The carburetor and idle adjustment screws will be located near the center of the engine as you look at it.
Turn the high speed screw (marked "H 3/4") counterclockwise until it stops, meaning three-quarters of a full turn.
Turn the low speed screw (marked "L 1") clockwise until it is fully seated. Following that, turn the screw counterclockwise one full turn.
Start the engine and allow it time to warm up.
Adjust the idle screw (marked "LA") by turning it either clockwise or counterclockwise until the cutting head does not rotate.
Determine your altitude above sea level. Your trimmer may require special fine-tuning adjustments.
Turn the high-speed screw approximately one-quarter turn for every 3,300 feet (1,000 meters) of altitude change.
Turn the high-speed screw clockwise to fine-tune it for high-altitude use. Continue turning it until there is no perceptible increase in engine speed. Perform this task with the engine running.
Turn the high-speed screw counterclockwise to fine-tune it for use at sea level. Continue gradually turning it until there is no perceptible increase in engine speed. Perform this task with the engine running.
Adjust the idle screw, turning it either clockwise or counterclockwise until the cutting head does not rotate.