Surging on a governed engine is always caused by insufficient fuel being available to maintain the governed speed.
So you either have a blockage in the fuel system, including the fuel tank & vent system or an air leak after the fuel has been added to the air stream going into the engine.
A small amount of water in the float bowl can also cause surging .
To eliminate air leaking in get a trigger pack of WD 40 or similar ( better value than a spray can in any case ) and with the engine running spray all around the carb manifold.
A properly sealed engine will show no difference.
For the rest is is a matter of cleaning and checking the tank can vent ( loosen the cap )