Many things can cause this symptom. First check your air filter to see if it is clogged. From there move to the fuel system. Be sure the carb is not being restricted by debris. Check your fuel lines for kinks or cracks and make sure the filter is still good.
Most likely it's a carburetion problem. Get a spray can of carburetor cleaner at any auto parts store, and clean the carburetor. It may also need adjustment, which can usually be done with just a screwdriver.
the govener that open the carb is bent or not working right so you need to take the top cover off and see if it broke or if it is bent or hanging up on something or dirt got in to it it should look like a plastic pices around the fly wheel