Turn the low-speed screw out a quarter turn.

Probably. Surging on a Stihl MS 261 usually means the low-speed mixture is too lean. Start by cleaning the air filter and checking for air leaks—cracked fuel lines, loose carb boot, or a dirty tank vent will cause surging too. If those check out, adjust the low-speed (L) screw counterclockwise in 1/8-turn increments. Let it idle for 30 seconds between tweaks.

Don’t touch the high-speed (H) screw unless you’re tuning under load with a tachometer. And don’t forget: the MS 261 has limiter caps. You’ll need the Stihl tool or a small screwdriver to get past