First, can you put the two mixture screws back where they were in the first place please? If you don't know where that was, we may have to do a start-from scratch tune when the fault is fixed. That isn't difficult, but if you can put things back as they were it will be a bit quicker.
The two most likely causes of your problem are first, a bad spark plug, or second, the wrong air/fuel ratio. I suggest you try a new spark plug, and if the problem persists, try removing the air cleaner in case it is blocked. If neither of those changes causes an improvement, clean and reinstall the air cleaner (for safety, in case the engine spits) and try loosening or removing the fuel tank cap (in case the cap vent is blocked. Be very careful not to spill fuel, of course - that stuff is dangerous, especially near a running spark-ignition engine.) All three of those possible causes are common every-day faults. In case you are wondering, your symptoms seem to match the third fault best, but I suggest you check these things in an orderly way in case you have more than one fault. (Vented caps on brush cutters have a one-way air valve, and during the off-season the valve often gets stuck shut when oil residue in it dries up or oxidises and becomes sticky.)
