Agreed, make sure you have the right size prop to begin with. The instructions should tell you what's suitable but a 10x6 or 11x4 would be fine and then there's no way it can rev too high. It also seems you may be starting it at full throttle which is quite ok to do but it can give a bit of a fright until you're used to them. Is the muffler fitted? An open exhaust is very loud and that can make the engine sound like it's running too fast as well.