Like most small engine carburetors, the one on the Stihl 021 chain saw has an adjustment for idle speed -- marked LA -- and one that controls the fuel/air mixture at low speed -- L. There is also a high-speed adjustment screw -- H -- but the company doesn't mention it in the user's manual because it isn't a good idea to adjust it yourself; if you set this screw improperly, the engine can burn out. The manual gives specific instructions for adjusting the other screws, though, and you should do it from time to time to improve performance.
Before You Make Adjustments
Remove the air filter cover and clean the air filter. You should follow a specific procedure for removing the air filter cover.
Depress the throttle trigger part way and turn the master control lever to "Cold Start." Turn the twist lock knob on the carburetor/air filter cover 90 degrees to unlock the cover, then lift it off.
Remove the air filter and clean it with a toothbrush or blow off the dirt with compressed air. Replace the filter if it's too dirty to clean.
Adjusting the Low-Speed and Idle Screws
Turn the low-speed screw, which is labeled "L", clockwise as far as it will go, using a flat-head screwdriver.
Turn the screw one full turn counterclockwise. Start the engine. You may now have to adjust the idle screw, which typically goes out of adjustment when you adjust the low-speed screw.
Increase the idle speed by turning the idle -- LA -- screw clockwise by a small amount. Keep adjusting the screw in this direction until the chain begins to turn, then back the screw off by one-quarter turn. When the idle is properly adjusted, the engine runs smoothly but the chain remains stationary.