The Troy-Bilt TB20CS string trimmer has a two-cycle engine, which uses a mixture of oil and gasoline to not only to ignite in the engine cylinder, but also to lubricate the internal engine components. As the engine ages, the carburetor needs adjustment to ensure optimal performance. If the engine runs erratically or will not start, adjusting the carburetor is the first step in the troubleshooting process. This procedure takes 10 minutes to complete.
Set the trimmer on a hard, flat surface, drain and refill the fuel tank with a fresh gas/oil mixture. Remove the air filter cover at the top of the engine; there are four screws holding the cover in place. Remove the air filter and inspect, clean or replace the air filter and reinstall the air filter cover, tightening down the four screws.
Locate the idle speed adjustment screw. On the top of the engine, there is a small hole that goes through the air filter/muffler cover, and through that hole is the idle adjustment screw. Start the engine and advance the throttle trigger to its highest position for a minute to allow the engine to get to operating temperature.
Release the throttle trigger to allow the engine to idle. If the engine stops while in the idle position, insert the flat-head screwdriver into the idle speed adjustment screw. Turn the idle speed adjustment screw clockwise in increments of 1/8 of a turn until the engine idles smoothly. If the trimmer's cutting head is rotating, move to the next step.
Turn the idle speed adjustment screw counter-clockwise 1/8 of a turn at a time just until the trimmer head stops rotating.