Not many string trimmer/brush cutters are designed to run a 50/1 mix...Most are 32/1 or 40/1. Only few "pro" units specify 50/1 mix... If ya change the mix ratio, then you are changeing the jetting requirements as well... And like he said, the idle mix screw is sealed on 99% of them... So stay with the mix ratio stated in the manual.. The symptoms he described, say he has a vacuum leak, otherwise the engine wouldn't "runaway" when he adds extra fuel by using the choke... If it was just a plugged fuel circuit, he wouldn't get the runaway symptom... USUALLY, on a weed whacker, the vibration will loosen the muffler screws.. But the leak could be anywhere, a loose carb, intake, case screw, crankseal, cylinder base gasket (if it has one), muffler, or head gasket... Unless it is one of the higher end "pro quality" units, Stihl, Husqvarna, McCulluch, Echo, then ya might as well see if you can RMA where ya purchased it... If not, buy another one...
The lesser quality units are disposable, not made to be repaired anyway....
Been racing 2stroke motorcycles for 40+ years... I've seized more than I can count, I know the symptoms!!!
Hard to diagnose by the BBs tho...
Hope it works out OK for ya..

RC