Look for a dirty/clogged air filter or fuel filter. A more likely cause would be a blockage inside the carburetor. Secondary, a bad gasket somewhere between the carb and engine block. A clogged fuel filter would allow the engine to run ok for a few minutes before surging, and a blocked air filter would cause a rich running condition.
Revving up and down is usually a symptom of a dirty air filter.
The governor spring adjustments
If you mean the engine will rev up and down as it is running, it has been my experience that a dirty air filter is a cause of the fluctuation.
The most common cause of engine surging (rev up and down) is a dirty air filter.
not getting gas. clean\carb rebulid needed
governor not moving freely or dirty carb
The govenor on the carbeurtor, When the motor is not getting enough fuel and is about to cut off, The govenor kicks in and squirts more fuel in to keep it alive.
Usually lack of oil or somebody put something in the gas.
it might men that your gas intake lever is sticky or rusted
blade needs sharpening,or belt is slipping for mower deck.
Rev limiter to save the engine from blowing up. Rev limiter to save the engine from blowing up.
your air/fuel ratio