How do I fix surging on a Stihl MS 261 chainsaw?

Turn the low-speed screw out a quarter turn. Surging usually means the low-speed mixture is too lean. First clean the air filter and check for air leaks—cracked fuel lines, loose carb boot, or a dirty tank vent. If those are fine, adjust the low-speed (L) screw counterclockwise in 1/8-turn increments, letting it idle for 30 seconds between tweaks. Don’t touch the high-speed (H) screw unless tuning under load with a tachometer, and remember the MS 261 has limiter caps that require the Stihl tool or a small screwdriver.