There is only one adjustment that you can make to the four-cycle Troy-Bilt trimmer carburetor. The high- and low-speed carburetor adjustments are preset at the factory. You will need to take the trimmer to an authorized dealer if these adjustments are necessary. Most carburetor problems however will only require the adjustment of the idle-speed screw, which you can perform on the trimmer yourself. Before making any carburetor adjustments, clean the air filter. A dirty air filter will cause the engine to run poorly and mirror a carburetor problem.
Locate the idle adjustment screw on the back of the Troy-Bilt trimmer near the carburetor. Start the trimmer and hold the throttle trigger to allow the engine to run at high idle for 2 minutes.
Release the idle trigger and allow the engine to come to its idle speed. If the engine stops you will need to adjust the idle screw clockwise to raise the idle speed.
Insert a small Phillips-head screwdriver into the idle adjustment screw head and turn the screw clockwise 1/8-turn.
Inspect the trimmer head for rotation while the trimmer is idling. If the trimmer head rotates while the trimmer is idling, you'll want to adjust the idle screw counterclockwise. Start the trimmer again and insert the Phillips-head screwdriver into the idle adjustment screw.
Turn the idle adjustment clockwise if the engine starts to cut off again. Continue rotating the screw in 1/8-turn intervals until the engine will idle and not shut off.
Look at the trimmer head for rotation as the trimmer is idling. Turn the idle adjustment screw counterclockwise until the trimmer head does not rotate and the engine stays running.