I am sorry you are having a problem with the mower. White smoke is caused by burning oil. I recommend checking the oil level first. If it is over full, that would cause the engine to smoke. To cure this you have to empty out the extra oil first. Once the oil level is adjusted, remove the air filter and discard it if it is oil soaked. Before installing the new filter try to start the mower. Once the engine starts, it will smoke for about five minutes. After the smoking stops, you can turn the engine off and reinstall the air filter. Now start the mower and check the operation.
Two things will cause this to happen. The first is over filling the oil. The second is tipping the mower the wrong way when cleaning under it. If you need to tip the mower to clean it, be sure to tip it with the air filter up and muffler down.
I hope this is helpful. I f you need further assistance, please reply below with more details and a model number.
Thank you for using Manage my life.