Fs 55 has very bad vibration

SOURCE: HOW TO GET BRUSH CUTTER OFF FS 55

Look at the top of the head (where the brush cutter is) find the small hole about 3/16 diameter. Insert an allen wrench or a large nail, or anything else that will fit in the hole almost the same size as the hole. While putting a little downward pressure on this "tool" turn you blade slowly by hand until you feel the tool slide into the locking slot, this will hold the blade from turning. Keeping it in this position, take a socket and ratchet on the nut that is holding the blade and loosen and remove it. THE NUT HAS LEFT HAND THREADS SO TURN IT OPPOSITE OF WHAT YOU NORMALLY WOULD TO REMOVE IT!!!!

SOURCE: Stihl FS 55 will not start

After trying to start several times, remove the spark plug and see if it is "wet" If it is you are flooding it. Do not pull the rope more than 3 or 4 times with the choke closed, whether it "pops" or not open the choke half way after the 4th pull and make sure you have the throttle trigger squeezed to give it gas