These little carbs are always a pain.... alot of times you spend more time fooling with them then they are worth. If the carb was all gummed up with "Old gas" u probably will need to install a rebuild kit. This kit will come with all of the little diaphram gaskets which are the important things in these lil guys. Most times when they become brittle they will cause the carb to not flow fuel like they should. Also if you spray the diaphrams with carb cleaner it will dry them out real quick (I've done that a few times)
That little brass screw and plastic plug really shouldn't be messed with too much. These carbs run so lean from the factory due to emmisions. But a lil adjustment shouldn't hurt
Also depending on what kind of oil was mixed with the gas check the exhaust screen in the muffler. Alot of times it will get clogged and cause the trimmer to run strange and not get up to full power.