- Try and squirt a little fuel in the carburetor.
- Clean the fuel tank and fuel lines
- Check the air filter is clean
If after trying above then check your stihl fs80 carburetor is not leaking and no fuel lines are leaking. Also download the free parts list PDF or try the service manual download from HERE
Some good carburetor guides can be found HERE
The stihl fs 80 trimmer carburetor could be clogged. Clogged carburetors happen a lot and are caused by leaving fuel in the string trimmer for a long period, as in your case. After awhile, some of the ingredients in the fuel may evaporate (yes it does happen), leaving behind a thicker, stickier substance, which the small fs80 carb cannot cope with.
This sticky fuel can clog up the carburetor and prevent the engine from starting. If the carburetor is clogged, try cleaning it with good quality carburetor cleaner. If cleaning the carburetor does not work, rebuild or consider replacing the entire carburetor.
Parts list, service manual and lots of carburetor service and rebuild articles are on this website.
Inspect the spark plug for signs of wear or damage. If the porcelain insulator is cracked, an electrode is burned away or damaged, or there is heavy carbon buildup at the electrode, replace the spark plug. To decide if the spark plug is defective, use a spark plug tester. You should see a strong spark between the tester’s terminals when the engine is cranking. If there is no spark, this indicates that the spark plug is defective and should be replaced.
To get this thing started, you can add a small amount of fuel down the throat of the carb after pulling the air filter. Do not, I repeat, do not use starting fluid, just your regular mixed gas, starting fluid is a bit strong and the carburetor acts like it’s addicted.
If if fires and dies after a bit, you can repeat the process,but generally speaking if after a few tries and it doesn’t run you have a blockage in something like the carb, in the filter, etc