The carburettor on a Stihl 021 has three screws that adjust the settings on the carburettor. These screws control the adjustments for the gas and air mixture in the carburettor at idle, low and high engine speeds. For your engine to perform optimally, the gas and air mixture must be set properly for all three speeds. While every chainsaw comes set from the factory, you may need to adjust the carburettor after working through as few as three tanks of gas, depending on your operating conditions.
Pop off the cylinder cover with your fingers. Pull out the air filter and slap it against your palm to get rid of any dust.
Drizzle about a capful of gas on the air filter band and pad. Slap them against your palm again to shake off any excess gas. Reinstall the air filter. Replace if the air filter is excessively dirty.
Empty any gas that has sat in the fuel tank for more than two weeks. Fill the tank with freshly mixed gas.
Start the chainsaw and let it idle for five minutes to warm up. Insert the tip of the small screwdriver into the "L" hole just above the clutch cover.
Turn the "L" screw clockwise so that it is seated all the way. Listen to the sound of the engine. Turn the screw back one full rotation counterclockwise. Listen to the sound of the engine again. This is the standard operating position.
Adjust the "L" screw using small turns either clockwise or counterclockwise, listening to the sound of the engine. Set the screw when you hear optimal engine sounds, which should be neither too high-pitched nor bubbly.
Insert the tip of the small screwdriver into the "LA" hole just above the clutch cover.
Turn the "LA" screw clockwise until the chain begins to spin. Back the screw off 1/4 turn counterclockwise, or until the chain stops spinning.
Adjust the "L" screw again using small turns either clockwise or counterclockwise.
It will take practice and careful observation to adjust the "L" and the "LA" screws to their optimal positions. Small turns on both screws can have a large impact on engine performance. Always monitor your engine and adjust accordingly.
It is not recommended to adjust the "H" speed screws, which if done improperly can permanently damage your engine.