Clogged airfilter/airbox perhaps....
That would hose yer mix, leading to the problems you describe as well, something worth checking...even if it is such a bitch to get the airbox out.
Back of the carb, the plastic piece attached there, it's supposed to mate to a plastic widget, you see, and it's damn hard to get that to HAPPEN, usually when yer putting the engine/trans assembly back in place....specially if yer liftin it with one hand and tryin to feed a bolt through with the other.
Vacuum leak as Chris stated is another, and my first suspect would be a loose manifold, if that's the widget between the cylinder and the carb, that is....hell if I know what half the parts are supposed to be called, just how to fix em.
Also, Tomos does have a rather longer warmup period than you might think, try leaving the choke on till the engine starts to fourcycle or lug, and THEN pop it off.....poppin off too early can also cause response like that.
And, this is weird, but every Tomos I've owned does it....start em frostbite-cold, fine....start em hot-as-hell, fine....try starting the damn thing warm...NADA, or the same things you guys are reportin - dunno what causes it, but I learned makin a short hop then shutting it off was a bad idea.
Kold/Arterious - try this...pull the throttle to about 1/2 WITH the choke on, and start it, run it for 30 seconds that way if possible, and if need be feathering the throttle (revving, whatever)...then slowly accellerate to full, then give it that last yank to pop the choke off...and see if it will keep going then.
If you know how to do a plug chop and it becomes possible to do so, do it, and lemme know if the mix is rich or lean, that'd help matters, to know.
Rich = more likely clogged: Lean=more likely vaccum leakage.
If that first trick works, then it's a matter of not leavin the choke on long enough for the engine to warm up all the way, and easily solved by simply including that in your startup process.