Lawn Mower Won't Start?

I have purchased a used, well kept Husqvarna self-propelled push lawn mower from a friend. It worked fine last year and this year I changed the dirty air filter, spark plug, and added gasoline and oil. It worked a couple of weeks ago and last night started in order for me to cut for 5 mins. or so. It puffed out white smoke and conked-out. I primed it, but to no avail, it will not start.