Mower surging / hunting at idle: it is the pilot jet, or an air leak

Surging (rev up, drop, rev up) is a lean condition at low speed. Causes in order of likelihood: plugged idle/pilot circuit in the carb (clean it, see the carb post), a cracked or missing intake gasket or primer bulb (spray carb cleaner around the intake while running; if the idle changes, that is your leak), a governor spring stretched or hooked in the wrong hole, or a worn throttle shaft letting air in.

Do not turn the governor screw to "fix" it; that just raises the RPM and hides the problem until the engine over-revs.