I have a stihl fs250 that i purchased new and went to using it on jobs, half way thru the 2nd year and running opti oil mix it up and quit running one day.I grabbed the back up and finished up, not knowing what the issue was i went to the dealer,an employee whispered to un screw the fitting on the muffler and see if it ran,it did and i was told this was common, 5-6 bucks later it was running with a new one and an extra one to clean for a back up. ( not a slide in screen type on the 250 ) The trimmer still has an issue about shutting off after a short while of running,?? is it possible the muffler could also have build up in it causing this ? or which way should i go.The mechanic told me that when i have a shut down problem i should disconnect the shut off switch wire at the coil and run it in case the wire has a short, i still feel that somewhere the carbon build up still has an issue,this year i will run amsoil at 80:1 to prevent this build up.any ideas on cleaning the muffler or where to go and look. I have picked up an fs80 and boy its light compared to the fs250 lol thinking the 250 will be the back up from now on.