white smoke almost always means your lawnmower is burning oil. this could mean either your piston rings are worn and need to be replace ( can be done easily if you have the tools) or you have too much oil in the motor.
White smoke is a sign that coolant is entering the combustion chamber. This is commonly caused by a cracked head or blown head gasket or both. This is a very serious condition that requires you STOP...
Steam?
Leak? overheating due to low coolant? Back pressure due to a bad thermostat? Defective radiator cap?
have system pressure tested to determine location of leak