According to what I read, the Troy-Bilt trimmer has an adjustable idle screw.
Use a small flat blade screwdriver inserted into a small hole, about the size of a pen tip, in the top right of the air filter cover.
Turn the screw clockwise 1/8 turn with engine running til it idles smoothly.
CCW 1/8 to stop the trimmer head from spinning at idle.
I don't own this trimmer so only telling you what I read.
My Ryobi wouldn't idle right, I discovered the head bolts had loosened after a couple years. Head actually was visibly moving up and down with each stroke of piston. Tightened 'em up and has run great since.
I've never used Stabil in my life and have always used up old gas at the start of every new season. But maybe your fuel mix itself is the problem especially since you put Stabil in it.
I'd mix up a new can with fresh gas and see if that works even before messing with the idle adjustment. And if so, next year, skip the Stabil.
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