Squeeze and release the primer bulb 10 times and ensure the gas tank has fuel if the trimmer refuses to start. Move the blue choke lever to position 3 and squeeze the trigger as you pull the starter rope if you suspect the reason it won't start is a flooded engine.
Replace the air filter or clean it off with warm soapy water if the engine won't idle. Adjust the carburetor by inserting a small Phillips screwdriver into the hole in the air filter/muffler cover. Turn the screw 1/8 turn clockwise at a time until the idle is smooth. Turn it 1/8 turn counterclockwise if the trimmer idles too fast.
Empty the fuel tank and refill it with new gas if the trimmer doesn't accelerate when you want it to. Clean out any grass, weeds or dirt that may be obstructing the cutting attachment.
Adjust the carburetor, replace the fuel or change out the spark plug if the trimmer lacks power or stalls when in use.
Replace the cutting line with a new spool or rewind the line if it doesn't advance from the reel as it should. Push in the bump at the bottom and pull out 4 inches of line to try again.