Looking for some help.

My son got an arctic cat 90 last year and we have always had a problem with it starting. I just replaced the plug and I am still not getting anything. Also I know that it has an auto choke but when I do get it started it runs for about 30 secs then cuts off. Then it is hard to get started again. I am not touching the throttle during start or idle. What could I be missing or what could be the problem?

Thanks......