just do process of elimination. check for air leaks to make sure you have none. if you dont, check your compression. if you have good compression check spark. you already said you have spark so you can skip that step. but then move to the carb. clean it out really good, and dont just spray carb cleaner through the air filter hole and say its clean. if it still does the same thing, check the petcock. if thats clean, clean your air filter. it should be running good by that point. also, it sounds like your jetting is off if you could run it around when it was cold, then it wouldnt run right when you tried to start it while it was hot