This site will give you a good, clear instruction on how to set the various control screws on a 2 stroke gas string trimmer. This one happens to reference an Echo brand of trimmer here in the U.S., but the settings are virtually the same for most brands.
As stated in the tutorial, the setting screws, although held in tension by their prings, tend to vibrate loose (or tighter sometimes) with use. I've found this to be the cause of rough running problems nearly 100% of the time...