I have an older Stihl FS55 purchased around 2002 or 2003. It has never ran well but was only used at my house so I never had it repaired. -There is no problem starting -idles rough and bogs down very quickly I own a tree care business and use stihl chainsaws. When they do not run correctly we adjust the carborator but this engine has no carb adjustment screws. There is an idle screw but it doesn't affect how the trimmer runs with the throttle engaged. I have a manual but it shows there being adjustment screws? there are the letters H and L on the carb but no screws? I am confused and unsure what might be the problem, any suggestions would be appreciated.