This will happen if the carby is sucking air. Check for cracks or if the wire plug (where the kill wires go) has been removed. It also happens if the throttle cable has come out of its guide. This can happen if the cable is bent at the ball end. On my mower I can tell when this happens as the throttle control "feels" loose or there is a jerk at the start and then it feels loose. The jerk is the ball popping out of the guide.
