Probably something caught in the carb, probably the main jet. Take the carb off and try to give it a cleaning. Use a good carb cleaner and compressed air to blow out dirt. It may be a problem with the governor, but without any info, it's hard to say. My engines class worked on an engine with the exact same problem. Cleaned the carb inside and out and the engine runs like new. Well, maybe not, but it does run much better.
JK, March 2008
