The Stihl 028 uses a carburettor and fuel system that can be adjusted to change the speeds of the engine. Three screws control three different speeds. The high, low and idle speeds can all be increased or decreased with these screws. By opening and closing vents inside the carburettor, more or less fuel will reach the carburettor, thereby controlling the engine speed. Adjusting of the carburettor is tricky, and improper adjustment can ruin the engine on the 028.
Unscrew the Stihl 028's fuel tank cap. Tilt the engine over the fuel container and dump out any fuel older than 72 hours. Fill the 028's fuel tank with freshly mixed chainsaw fuel.
Unscrew the back cylinder cover knob with your fingers, and lift the cylinder cover off the engine. Unscrew the air filter from its mounting posts and separate the pad from the air filter box. Brush the air filter, air filter box and the area around the air filter. Slap the pad against the palm of your hand to shake off any dust.
Unscrew the front muffler cover and unscrew the spark arrester screen. Brush the metal screen and inside the muffler. Put the screen and muffler back together. Place the 028 on the ground, start the engine and let it idle for two minutes to warm up.
Locate the carburettor adjusting screw holes on the side of the clutch. Insert the small screwdriver into the hole marked "LA" for the idle speed. Turn the "LA" screw clockwise until the chain and clutch start spinning on their own. Turn the "LA" screw counterclockwise slowly until the chain stops spinning.
Insert the small screwdriver into the "L" hole for the low speed. Rotate the "L" screw clockwise until the engine sounds like it's starting to surge. Rotate the screw counterclockwise until the engine sounds like it's starting to bubble. Move the "L" screw between the surging and bubbling sounds to find the cleanest engine and best low speed setting.
Throttle the trigger and make sure the engine accelerates properly. Turn the "L" screw in 1/8-turn if it's sluggish. Readjust the idle speed in the same manner as before, because the idle speed will have changed slightly after adjusting the low speed.
Don't attempt to adjust the "H" or high speed screw without professional knowledge. Improper carburettor adjustment will destroy the engine.