Stihl uses an all-position diaphragm carburettor on most of its engines. These carburettors have intake and outtake valves, which direct the flow of fuel through the mixing chamber. When these valves, and the filters inside them, get blocked with dirt or dried gas, you will experience hard or erratic starts, surging engines, slow acceleration and poor performance under full load. Removing these obstructions should be the main priority when cleaning these carburettors. Leave disassembling and reassembling of the carburettor to qualified professionals.
Unscrew the air filter cover or lift out the tabs with your hand. Remove the air filter cover. Take the air filter off its mounting plate. Pour about two capfuls of fuel onto the air filter.
Rub the fuel into the air filter to clean it. Wipe it dry with the rag. Slap the air filter against your palm to remove any excess fuel. Reinstall the air filter.
Fill the fuel tank with freshly mixed two-cycle fuel. Screw the gas cap back in place. Set the choke to its middle setting, so it's half-open. Locate the carburettor's half-opened neck.
Place the extended nozzle onto the carburettor cleaner. Pull on the starter rope to start the engine. Spray a one-second blast into the open neck. If the engine dies, let it. If it continues to run, depress the throttle until the carb cleaner burns off the build-up inside.
Restart the engine if it died. Depress the throttle until all of the white smoke clears to clean your carburettor. Use the carburettor cleaner every one to one weeks.
If you can't start your engine after using the carburettor cleaner, then your carburettor will need to be disassembled, cleaned and reassembled with a carb kit.
Disassembling your carburettor may void your manufacturer's warranty.