You should clean the air filter after every 10 hours of use.
The trimmer's cutting attachment -- the string -- continues to spin while you are adjusting the idle speed. Wear protective clothing and keep a safe distance from the attachment while adjusting the speed.
Many of the problems you encounter while using a Troy-Bilt "weed trimmer" can be attributed to a dirty air filter, a bad spark plug or a poorly adjusted carburetor. Some of the problems they can cause include the trimmer not idling properly, the engine not starting and the engine stalling while in use. Fortunately, cleaning the filter, replacing the plug and adjusting the carburetor are all things you can do on your own.
Set the choke lever to "Position 2" and use a flat blade or T20 Torx bit screwdriver to unscrew the four screws keeping the air filter cover in place.
Remove the air filter cover from the engine. Do not force the cover off, to avoid damage to the cover or engine.
Turn the cover over and remove the air filter.
Use water and detergent to wash the air filter. Once the filter is washed, squeeze it to remove any excess water. Allow the filter to dry.
Add just enough SAE 30 oil to lightly coat the filter, and then squeeze the filter to ensure that the entire filter is covered.
Place the air filter back into the cover, and then put the cover back in place on the engine. Tightly screw the four screws to secure the cover.
Wait for the trimmer's engine to cool. Once the engine is cool, find the spark plug on the trimmer's cylinder head. Grab the spark plug wire and remove it from the spark plug.
Use a cloth to clean the area around the spark plug. Take a 5/8-inch socket and use it to turn the spark plug counterclockwise, until the plug comes off.
Use a feeler gauge to set the replacement plug's air gap to 0.020 inches.
Place the new plug on the cylinder head and use a 5/8-inch socket to turn the plug clockwise, until it is securely in place. Take care not to tighten it too much.
Start the trimmer's engine and run it at a high idle to allow it to warm up.
Let the trimmer's throttle go. If the engine stops, use a small Phillips screwdriver to adjust the idle speed screw in the air filter cover.
Turn the screw 1/8 of a turn in a clockwise direction.
Start the engine and then release the throttle. If the engine stops again, turn the screw another 1/8 of a turn. Do this process until the engine runs idle instead of stopping.