Most Troy-Bilt trimmers use a centrifugal clutch system, which, when the engine reaches a high enough speed, the force generated pushes the clutch outward. This engages the drive shaft. These clutch systems tend to wear out after several seasons if you don't clean and maintain them. You may notice grinding or high-pitched whining noises emitted from the area just in front of the power head: these noises indicated clutch drum is thinning and almost gone; replace the clutch immediately should you hear these sounds.
Shut down the Troy-Bilt engine by moving the ignition switch to the "Off" setting. Lift the rubber boot off the spark plug, which on Troy-Bilt trimmers will generally be on the underside of the engine.
Unscrew the bolts holding the throttle control to the rear handle on the trimmer shaft. Separate the throttle control halves and remove from the shaft. Unscrew the air filter and muffler cover on the rear of the engine. Take the cover off and lift out the air filter.
Disconnect the throttle cable from the carburetor by squeezing the clips inward needle nosed pliers and pulling it backward. Disconnect the two lead ignition wires from the ignition switch on the throttle control/ignition switch.
Undo all the screws at the front of the power head.These hold the shaft to the engine. Remove the screws and withdraw the shaft from the front of the power head. Remove the front shroud covering the starter and clutch assembly.
Pull out enough starter rope and feed it into the open cylinder hole where the spark plug connects. Feed about three inches of rope slowly into the hole to block the piston. Unscrew the clutch's hub with a socket wrench and 5/8-inch socket. Remove the clutch drum, clutch and washer.
Clean the parts with a rag and dish soap. Inspect the clutch drum for signs of thinning, cracking and damage. Replace clutch parts as necessary. Install parts in reverse order: Washer goes onto the rotor, clutch assembly on top and then the clutch drum; tighten them down with a socket wrench.
Remove the starter rope slowly; be sure you get it all out. Fit the shroud to the front of the clutch, tightening all the bolts to secure it in place. Push the shaft into its seat on the housing and tighten the remaining screws. Reconnect the throttle cable to the carburetor. Reconnect the ignition wires to the stop switch. Refit the throttle control to the rear handle and lock it in place. Reinstall the plug and boot to the top of the plug.