Many different problems can cause your Stihl trimmer to stop starting properly. The trimmer needs a consistent influx of fuel and air, at just the right mixture, at just the right time. This complex system of air, fuel and spark, if off even slightly will cause starting problems for your trimmer. Keeping these three working properly will solve most minor engine problems.
Dirty Air Vents
Like the human breathing system, your engine pulls fresh air in and pushes dirty air back out. The air filter and the surrounding area must be free of particulates to allow the intake of air and the muffler, spark arrester screen, and exhaust port must be free of carbon build-up. When these air vents clog up, you'll notice the engine trying to start but dying soon after. Clean the air filter and the area around it after every one to two uses. The area inside the muffler needs cleaning every 20 to 30 hours. You also need to replace filters and screens before they start causing problems, as they can lead to trouble in other, more sensitive engine areas.
Bad or No Spark
Troubles with the ignition system will cause erratic starts, hard starts and the engine will seem to run roughly. The ignition module connects to the flywheel and when the engine reaches speed, a high voltage spark is sent up to the plug for combustion. This all needs to happen at the precise moment with the precise amount of voltage. Dirty, badly gapped, or corroded plugs will stop the spark from discharging. Change your spark plugs seasonally at a minimum or more frequently, depending on usage. If you're still having spark problems, take it to a professional to hook up an ignition system tester. This will machine will determine where your spark is getting lost at.
Blocked Fuel System
The most common starting problems usually deal with a blocked fuel system. The gas needs to flow from the tank to the carburettor, from the carburettor to the chamber, and any excess must get purged back into the tank. Dirty tanks, tank vent holes, plugged fuel filters, dirty fuel lines and improperly tuned carburettors will cause a wide array of problems. The pump inside the carburettor on these trimmers tends to warp and cause problems after four to five seasons. Taking the trimmer to a mechanic to clean the carburettor seasonally will extend the life of your Stihl carburettor.
Internal Engine Problems
The most serious starting problems deal with the piston, cylinder and crankcase. If air has leaked into the system, you won't get the trimmer started. Bad gaskets, seals and piston rings will cause air leaks. A sheared piston will also stop your engine completely. These problems are difficult and expensive to fix and may even outweigh the costs of a new trimmer.