First, I would suggest next time have a more descriptive title and place it in the 4 stroke section if you'd like more responses to your question.
Also, state as much about the engine as you can: approximately how many miles you have on the engine, what modifications you've done to the engine, if you have a fuel filter and/or if you changed it, if you've cleaned the air filter, adjusted valves, etc.
Is the choke on or stuck open? Do you have a fuel filter? It is a possibility that the slow jet is clogged. Here's a video on how to clean it:
http://www.youtube.com/watch?v=QtBl63kWO7Q
Good Luck,
Chris
AKA: BigBlue