Hard starting and poor running

Question

After replacing the filter & plug, and adding fresh gas, my toro mower w/ a 6 hp Briggs engine just won't run well. At idle and at speed, load or no load, it constantly varies in rpm and starts v. hard. Never a problem until this year, so I'm stumped, especially since I see no idle adjusting s***w on the carb.Thanks in advance for any help or tips.Frank