The throttle screws on a Stihl chainsaw control the air and fuel mixture that passes through the saw’s carburettor. With the optimal throttle settings, the saw will produce a clean sound when running, won’t die or move the chain while idling and will cut wood quickly. On Stihl saws there are two throttle or carburettor adjustment screws. There is a low-speed adjustment screw, which controls the amount of fuel in the air and fuel mixture while the saw is idling. The high-speed adjustment screw controls the amount of fuel in the air and fuel mixture when the saw is cutting. Fine-tuning adjustment is needed to maximise the saw’s performance.
Remove and clean the air filter. Make sure the saw’s gas tank is at least half-full with fresh gas. If the saw is cold, start the engine and run it for a few minutes to warm it up.
Locate the three holes on the right side of the Stihl’s body, near the handle bar. The top one is the high-speed adjustment, the middle is the low-speed adjustment and the bottom is the idle speed.
Set the idle speed. This is the bottom screw, labelled "LA." Screw the idle adjustment clockwise until the saw dies. Turn the screw back counterclockwise one quarter of a turn.
Start the saw again and let it idle. Set the low-speed adjustment, or the middle screw, by turning it clockwise until the engine surges or sounds choked of fuel with a high-pitched wine. Remember the position of the screw in this setting.
Turn the adjustment screw counterclockwise, until the engine starts making a heavy bubbling sound. Remember the position of the screw in this setting.
Turn the screw back slowly clockwise to where the idle sounds the best and cleanest. This position is roughly halfway between the tightest and the loosest the screw can get before the engine dies.
Reset the idle speed, or the bottom screw, because the gas and air mixture will have changed with the low-speed adjustment. If the idle speed is properly set, the chain should not move while idling and shouldn’t die when not throttled.
Hold down the saw’s throttle so it is at its maximum speed. Set the high-speed adjustment screw by turning the screw clockwise until it slows down and starts to make a bubbling sound. Remember the position of the screw in this setting.
Rev the throttle to its maximum again and adjust the screw counterclockwise until it produces a high, fast and clean sound. Remember the position of the screw in this setting.
Turn the screw back slowly clockwise until the saw has a rougher, gurgling sound to the engine. This position is about halfway between the tightest and the loosest the screw can get before the engine dies.
Reset the idle speed and low-speed adjustment because they will have changed after setting the high-speed adjustment screw.
It is unwise to adjust the maximum throttle speed or high-speed counterclockwise adjustment without a tachometer. Adjusting the maximum speed too high can permanently damage the engine. Always wear the proper personal protective equipment when working with saws.