The carburetor may be getting too much fuel, this can happen if the carburetor float is stuck in the open position or the carburetor choke plate is stuck closed not allowing the engine to draw in enough air to create the proper air fuel mixture. You can try cleaning the carburetor with the carburetor cleaner to ensure the float or choke plate can move freely. You can also purchase a carburetor repair kit to replace corroded or damaged components or install a new carburetor altogether.
The engine piston rings could be worn and allowing oil to seep past and be burned in the combustion chamber. You can replace the old piston rings with new ones to resolve the issue.
03 - Adding too much oil
The engine crank case could also be filled with too much oil. Adding more oil than is necessary can create extra pressure in the crank case, causing the seals to blow out and allowing the oil to migrate into the cylinder head where it can produce excessive exhaust smoke. Always follow manufacturer’s recommendations for the proper amount of oil to add to the mower’s engine.