The most common reasons for a surging
Bad spark plug
Dirty / faulty carburetor
Fuel filter blocked
Fuel tank blocked
Fuel line blocked
Gasket vacuum leak
Manifold vacuum leak
Governor control fault
You can apply a half choke with the engine running if you have a motor without a choke holding a finger half over
the carby intake will do once the air filter is removed.
If the engine now runs without surging – Gas starvation is the likely fault. If it runs just the same .A vacuum leak is the more likely fault.