what to check/replace to make engine run smooth?

My lawn mower (has a B&S 3.5 h.p. engine) runs, but sounds like it has a miss, does not run at a constant speed. what (and how) do I check i.e. points, condenser, etc? The spark plug is brand new (less than 20 minutes running time) so I am sure that is not the problem. It is a Murray 20", B&S 3.5 hp. "classic" with throttle-free operation...carburetor was replaced earlier this season, and all ran fine till the last time I cut the grass.