When your Stihl trimmer stops running and nothing you do seems to help can be quite frustrating and you wonder where you can start with all those parts. Basic maintenance and cleaning should always be the first place you start. Other problems you can classify and diagnose into one of three engine areas: fuel, spark or compression. Deciding whether you should fix them or take the Stihl to a service dealer will depend on your level of experience with small engines.
Cleaning and Maintenance
Sounds simple, but many problems can be solved with 10 to 20 minutes of cleaning and maintenance. Start with your fuel tank, making sure it has enough freshly mixed fuel that meets the operating standards for Stihl trimmers: old or bad gas cause a majority of all engine problems.
Drain and clean the fuel tank, and check the fuel filter and fuel line to make sure there are no holes, kinks or tears in the plastic. Moving on to your air filter, you should wash the filter in warm, soapy water and clean the surrounding area thoroughly with a brush and rag. Let the filter dry before reusing it. On the other end of the air, open up your muffler and clean out the muffler, exhaust port and spark arrestor screen.
Replace your Stihl trimmer's spark plug with a new Stihl spark plug. Wearing leather gloves, hold the plug, with attached boot and wire next to a grounded metal spot on the engine block. Crank on the starter rope a few times and you should notice a blue spark across the metal points. If there's no spark or it's weak, you'll need to work back from the plug wire to the ignition module, checking all connections, wires and switches for loose points and damage. If there's still no spark you will probably need a new ignition module.
If you have a spark and there's clean, freshly mixed fuel in the tank you are likely getting insufficient gas to the carburetor. Fuel lines, fuel filters and primer bulbs need replacing every season or sooner if you left gas in the tank over winter. Fuel will degrade the plastic lines and suffocate the carburetor. If your machine is a few years old, you will need to remove and clean out the valves, screens and pump on your carburetor.
If you still can't get your trimmer to start and run, you likely have an air leak somewhere in the system. If extra air gets into the engine the fuel mixture will be off and the engine won't start and run properly. These problems, related to compression, will be deep inside the engine, on the piston seals, gaskets or inside the cylinder, which will require a professional to replace.