Weedeaters are small, two-cycle engines that run a small head with blades attached. These blades, or thin wires, are used to trim sections of your lawn that are not easily reached with a traditional lawnmower, and for 'edging' areas near sidewalks and the house. Weedeaters, due to their small engines, are easier to maintain and fix than any other piece of lawn mowing equipment, but do require a knowledge of good troubleshooting techniques to isolate the initial issue.
Check that there is sufficient gas and oil in the engine of the weedeater if the engine will not start. Most Stihl models require a mixture of oil and fuel for the engine to operate. Fill according to the mix required of your model.
Make sure the carburetor is fully choked when trying to start the engine to prevent it from seizing. A partially choked carburetor will not allow the engine to start.
Use a screwdriver to adjust the carburetor if the engine is starting and then dying immediately, or not starting at all. Carburetors have two screws, one for the idle speed of the engine and one for the high speed. Adjust both screws using a screwdriver so that they are in the middle speed. Stihl weedeaters should be adjusted by 3/4 turns.
Replace the air filter if the weedeater is running at low speed or power. Remove the screw in the middle of your air filter housing--on a Stihl it is a flathead screw--and take off the housing. Remove the air filter and replace it with a new one.
Check the spark plug, if the engine will not start, to make sure it is free of dirt and that it is in working order. Look at the electrode gap and make sure it is adjusted properly to the weedeater model you are using.