Craftsman Lown Mower 24 hp Briggs Stratton Engine, Model 917.288560

When engine runs, it surges in low and high rpm. We cleaned the carburetor, the fuel filter and set the valves. It starts up very easily but still surges a little and appears to be running lean. I do not want to cut the lawn for fear of running it hot and causing an overheat. Thank you for your help. MaryAnn and Frank