plumes of white smoke comes out of my riding lawnmower while i'm mowing. it happened twice in a hour. then it

Answer 1

Grey smoke (white) is an oil leak.
Black smoke is a fuel leak.

Answer 2

White is usually water,you may have a little water in the gas it will collect in the carburator float bowl and pick up small amounts as you drive it.Eventually it will accumulate too much water and it will quit running.

Answer 3

Might have oil leaking somewhere. Have you tipped it over at all. Sometimes the oil will leak out then