2008 big bear starting issues

I have a 2008 big bear. Purchase April 2009, now has 400 miles on it. After it sits for a little while (30 min or so) after a ride, when I try to start it, it back fires in the carb and just turns over and wont start. I have to turn the gas off, turn the engine over for a while and then it finally starts up and then i turn the gas back on and it runs fine. the spark plug is sooty. i dont think its bad gas because i have been running on the same gas for about 2 days.

any help appreciated

you may need to get a valve tune up or just a dirty carburetor check filter as well change your oil often