How can i fix my STIHL FS55 from engaging gear immediately I fire it up?

My machine starts easily but as soon as it does, the wire starts whirring and the throttle appears to be stuck at full throttle for the whole duration I am using it. Today I did over an hours strimming (one tank full) and my hand/fingers were tingling having used the machine at full throttle all this time. I have tried WD40. I cannot strip it down as I do not have a screwdriver to fit. Is it a TORX screw?

Margaret Elliott
June 2009