I brought an fs55 stihl strimmer about 18 months ago, it has never been a great starter but this season things have got much worse after alot of pulling I can eventually get it to run but after about twenty minutes it will cut out and not restart, I have various other two stroke tools for which i use the same 2 stroke mix without any probs so i know fuel is not the problem.
take out the spark plug and check for a spark hold the plug with insulated pliers whilst cranking if no spark get the coil tested most decent mower shops will charge about £5 for this no need to remove coil at this stage good luck