The carburettor that comes in Stihl chainsaws is commonly referred to as an all-position diaphragm carburettor. These carburettors use an integral fuel pump, which is separate but attached to the back of the carburettor. Stihl manufactures its own carburettors and sets them at the factory to specific settings for Stihl chainsaws.
The idle-speed screw on Stihl carburettors controls the speed at which the engine idles. Usually around 2600 to 2900rpm, the idle speed should be adjusted regularly to ensure the optimal point for engine acceleration. Turning the screw marked with an "LA" just above the clutch cover can set this idle speed. The settings for this screw should be at the highest engine speed before the chain begins spinning when the trigger isn't throttled. Never operate your Stihl chainsaw if the chain spins while idling; this is a dangerous situation for the cutter. Immediately adjust the idle screw until the chain stops spinning.
The low-speed screw adjusts the amount of air and fuel in the carburettor's mixing chamber at low speeds, usually below 10,000rpm. This jet inside the carburettor comes set at the factory, but, due to gasoline impurities, will need adjusting to achieve the optimum air and fuel mixture. You can adjust this setting by turning the screw marked with an "L" just above the idle screw. You can find the factory setting for this screw by turning it clockwise until it's seated; then, turn the screw back counterclockwise one full rotation to reach the "1" position. Only adjust the screw using small turns, which will have a noticeable impact on engine performance.
The high-speed screw regulates the maximum engine speed. This screw controls the high-speed jet, which adjusts the air and fuel mixture at the top engine speeds. Due to the danger of improper adjustment, it's better to leave this adjustment to professionals. The maximum engine speed on most Stihl chainsaws is 13,000 to 14,000rpm, and the only way to achieve this setting is to use a tachometer while adjusting the high-speed screw. The factory setting is achieved in the same manner as the low-speed screw, and the proper setting is "1" or one full turn open from its seated position.
The carburettor needs to maintain a specific level of pressure to operate continuously. If the pressure is too weak or fluctuates, the engine will shut off. This is a common problem when the engine shuts off after it is hot. To test your Stihl's pressure you will need to perform a pressure test with a specific gauge. This requires carburettor removal so it's best to leave this test to professionals. The proper pressure for Stihl carburettors varies from model to model, but is usually around 5.26kg. per square inch.