How old is your lawnmower? How long ago was the carb cleaned out?

Hmmm. '90s 'F' engine. One thing for sure, it doesn't need 're-jetting'. There are two causes; one, it has started to run lean, and/or two, a problem in the governor.

I have a 1994 model 10520 gold series. The carburetor is adjustable.

The first thing that I would try, to eliminate the surging, would be to add a small can of carb cleaner to the 5 gallon gas can used to refuel this machine. There could be small deposits of gum/varnish in the carb, that somehow were missed when the carb was cleaned. Our mower had the same problem this spring, and that was the solution. It takes two or three hours of operation for the surging to go away entirely.