you've probably got an air leak or a dirty carb. pull the carb off, and clean out all the passages. check for leaks around the head, where the jug meets the bottom end, and the reed cage. sometimes there's a leak at the top of the carb as well
with the motor running spray carb cleaner around the areas i described, if it revs up then its got an air leak there. otherwise you can spray some soapy water around there and kick it over and look for bubbles