Fuel and breathing is often the culprit but you seem to have gone throug that.
May I suggest that you check the ignition cable? It could be almost off inside the rubber and that can cause it to run but badly, a bit like you describe it.
I had a cheap curve trimmer that was hard starting, revved slowly, used a lot of fuel and suddenly stopped and lost spark. Checked the on/off switch wiring and all else in the ignition and found out that the wire was broken 15mm into the rubber. Cut the cable and soldered it again and it runs like a champ since then (like 10 yrs) and cant believe it is a china trimmer