Gas weedeaters can be a pain. I'm far from being an expert on them, but I've kept a few running a long time (along with several dirt bikes and other 2-cycle engines). Wild ass theory: The old mix might have separated a little bit (and some of the gas may have evaporated) while sitting for a long time and after running it in the front yard the ratio could have changed to where there's too much oil in the remaining gas. This could be the source of the smoke and maybe the need to run it on the B choke setting. The too high oil ratio + half choke setting would make it barely run.
I'd check the spark plug (clean or replace, check the gap), put new pre-mix in, clean the air filter, and see how it runs. If it still requires running with the choke on, I'd fiddle with the carb until I fixed it or made it worse (mumble a few choice words). Check your manual to see what kind of adjustments it has and try to match the factory settings. If it doesn't have a low adjustment, there may not be much you can easily do. With a 2-cycle some light smoke here and there (slightly rich) is no big deal, in fact it's much better than running too lean (and it helps keep the mosquitoes away). I currently use a Stihl that smokes a little, but it runs fine.
The little 2-cycles are very finicky and quick to let you know if the gas is old, dirty, mixed wrong, has water in it, etc. Good luck.
