the engine could be getting to much fuel causing a flooding or the air filter could be blocked causing the engine to choke up or the spark plug could be goosed so it isnt firing the petrol all the time or your petrol could be old and not firing properly
It is probably the governor or the carburetor. Clean the carburetor inside and outside and the governor and all linkages with carburetor cleaner. If this doesn't fix it, try adjusting the adjustment screws on the carburetor with the engine running.