Start with the basics. Make sure the ignition switch is on. Make sure the fuel tank is full and has the proper mix of oil and gasoline and that the bar oiler tank is full with the right lubricant. Make sure the chain brake, a handle in front of the main saw handle, is properly engaged. Test the chain to make sure it will turn freely. Adjust the tension if necessary; it should be tight, but loose enough that the top section can be lifted until the teeth just fit inside the groove in the bar. Adjust it by loosening the retaining nuts with the scrench --- combination screwdriver/wrench tool --- and turning the adjusting screw with the scrench.
Review the starting procedure to make sure all the steps were followed properly. Remove the cover and inspect the air filter; if it is dirty, clean it by washing it in mild soapy water and then drying it thoroughly before replacing it. Install a new filter if the old one will not clean thoroughly. Clean any dust or grime from the cover before replacing it.
Check the spark plug; remove it with the scrench. See if it is fouled with carbon or is wet with gasoline. Clean off carbon with a small wire brush or very fine sandpaper. Check the gap, which should be 0.5 millimeters, and adjust it if necessary. Change the plug if it is very dirty or has had approximately 100 hours of operation. If the plug tip is wet, let it dry completely before replacing it, and let the carburetor air out to remove any flooding fuel.
Remove the bar --- loosen the nuts holding it on with the scrench --- and check the chain groove. Clean it with a fine wire to remove any debris. Make sure there are no burrs or broken spots. Inspect the drive teeth on the bottom of the chain to make sure none are worn or damaged. Examine the drive sprocket to make sure it is not damaged. Check the oil holes in the bar, and inspect the feeder holes in the saw body to make sure nothing is plugged and oil can flow freely into the chain groove. Clean the cooling inlets, cylinder fins and spark arresting screen behind the muffler.
Adjust the carburetor if the engine is not idling properly. Turn the top idle screw --- marked "L" --- on the carburetor counterclockwise with the scrench until the engine runs smoothly. If the engine stops while idling, turn the idle speed adjusting screw, marked "LA," or the bottom of the two adjusting screws, clockwise until the chain begins to turn, and then back it off one quarter-turn. If the chain turns while the engine is idling, turn the idle adjusting screw counterclockwise until the chain stops, and then turn it another quarter-turn.
Inspect the chain teeth to make sure they are sharp if the saw is not cutting or is cutting at an angle, which indicates unbalanced teeth. Sharpen the teeth with a round file. Use a Stihl filing gauge, and follow its directions for the size of file and angle of cut, which will vary with the type of chain used. Use only forward strokes, no more than two or three per tooth. File from the inside of the tooth to the outside, and lift the file out before starting the next stroke.