Adjusting a sthil fs 75 carburetor? fairly easy if you try these steps:
Turn the high-speed screw, marked ‘H’ counterclockwise until it stops.
Tighten the ‘L’ screw all the way, then turn it one turn counterclockwise to open it.
Start up the engine and allow it to warm up. The cutting tool will rotate. Adjust the ‘L’ screw clockwise until the cutting tool stops moving.
Press the trigger of the machine to rev the engine. Adjust the ‘H’ screw until the engine does not cut out or sputter.
The carburetor is factory-adjusted for sea level, and if you use the trimmer at high altitudes, it may not get enough air for proper combustion. Most Stihl trimmer carburetors have adjustment screws for high- and low-speed operation, as well as an idle adjustment. The manufacturer’s recommended adjustment procedure is to screw both the high-speed screw — “H” — and low-speed screw — “L” — clockwise until they seat, then back each off a complete turn. After that, adjust the “H” screw no more than one-quarter turn in either direction. Make the adjustments while the engine is warm.
As I remember, hesitation on acceleration indicates a main mixture that is slightly lean. If you have a high speed needle adjustment (usually indicated with an “H”) and it is not already up against a limit stop, I’d try opening it a little. Say 1/8 turn or so to see if that helps. I’d avoid taking the carb apart if possible. If you do decide to take it apart, I’d advise having a rebuild kit available before you do.