Put some fresh fuel in it and prime the heck out of it to flush the carburetor. Also try speeding up the idle, the lower adjustment screw,to just below where the head starts to turn.

If it bogs and dies, then canguy's idea is good, but if it just refuses to speed up, take the spark arrestor
screen out of the muffler and burn the carbon out of it.
Post your model number for more specifics.

Fish

Sounds like the spark arrestor. Look at the muffler where the exhaust comes out. You will either see a plate held on with two screws under which is the screen or you might see a hex nut which you unscrew.

Heat the screen area till it gets red hot with a propane torch and use a brush to remove the deposits after it cools.

Or it could be the spark areestor................

Fish

how are you getting on? ive had the same trouble with a stihl fs76 trimmer. some people have suggested air leaks in the fuel line and others have suggested carb blockages. the priming trick worked well for me although it went back to being crap again after a few minutes. im stihl scratching my head on this one.

Same answer; check the spark arrestor first.

If the trimmer ran fine with your fuel mixture screws set to where they are now, you really shouldn't have to mess with the fuel mixture screws. If it bogs and dies when you give it part throttle, does it run a little better with the choke half way? If so, take the carb off and clean it out with carb cleaner and put on a new fuel filter. If it still won't run, clean the spark arrester.

I just bought a stihl. The dealer told me that the fuel mix (using the Stihl oil) was good for 6 weeks 8 weeks max. Anything longer will cause you problems. Can't confirm or deny, just passing it on.

Good Luck,

Dave

The Stihl oil contains a stabilizer that should keep the gas in good condition for a lot longer than that but I like to get fresh minimum once per season. The dealer is right about untreated fuel.

I think Fish is right on. Try cleaning the spark arrestor as he suggested.