knocking

Hey guys. I'm 17 years old and this is my first build. I'm building a sort of board track racer style bike, but I'm at a stand still right now because I can't get the engine to run right. I bought the black 48 cc engine kit from luckyearlybird on ebay. When I first put it together, it...

when i get into higher speeds on my 80cc beach cruiser, I hear a fast knocking sound and would like to know if I should be worried about that? or what is causing it?