A Troy-Bilt trimmer is a great power tool--until it won't start. Many things will cause your trimmer not to start and isolating the specific problem can seem difficult. However, understanding that only three things are necessary to start, will give you a base point to start working with.
Check for Fuel
It may sound silly, but fuel problems cause a majority of starting problems. Mixed trimmer fuel goes bad after 30 days, so if old gas is still in the tank, dump it out and mix up a fresh batch. If the fuel is OK, dump it out anyway and check the tank, fuel filter and fuel lines. Often, dirt, weeds or other debris will block up the fuel system. Remove these pieces and replace them if they're suspect or if they haven't been replaced this season.
Check for Spark
A bad spark plug won't deliver the energy needed for ignition. Unhook the black rubber boot and unscrew the spark plug. Check the tip and if it looks black or dirty, replace it with another trimmer spark plug. Set the new plug back into the rubber boot so the metal tip points out. Put the trimmer on the ground brace the shaft with your foot and hold the tip of the plug about a half inch away from a metal spot on the engine block that runs into the ground. Pull on the starter rope and look for a blue spark. If you don't see a spark or it's yellow, something's wrong with the ignition system. Take the trimmer to a professional to work on the ignition, as the voltage here can be lethal.
Check for Air
Open up the side air filter box and take out the air filter. Dirty filters account for a lot of starting problems because it's like a set of hands squeezing the air out of your trimmer's lungs. Wash the pad in warm, soapy water and let it dry out overnight or replace the filter if it's too dirty to clean. After 60 hours of operation, check the muffler, spark arrestor screen and exhaust port. Clean if necessary because build up here will cause a lot of starting problems.
Check for Compression
Pulling out on the starter rope, notice if it seems too hard or too easy to pull. Both result from compression problems. If you suspect compression, hook up a compression gauge to the cylinder—where the spark plug fits in—and pump up the pressure. Watch your reading; it should stay consistent for at least a minute. If your pressure falls off, you have air leaking somewhere in the engine. Leave these repairs to a professional as well.