I think you may have problem with how you preceive smooth running, under load the mower engine should smooth out, at idle the mower should sound slightly rough and blubbler a little ( sould like a idleing line trimmer ,old dirt bike or big chainsaw)
I have sold about 100 of these suzuki engine 2 stoke commerial mowers by Toro. as long as the carb does not leak and the engine does not die (I think it idles about 600 rpm).. it is fine.
Good Luck,
Friiy
P.S. make sure the engine does not surge when idling or hesitate when throttled up. Check the shaft, seal and hole for the govener shaft comming through the engine. they erode bad from grit and dirt from mowing ( they suck air and wear out the adjustment in the linkage)