Mini atv pocket 49cc wont stay ticking over ?????

hi there my mini atv quad bike wont stay tick over after my chain had snaped and sometimes leaks petrol from the over flow pipe when its not even over flowed ive tryed tuning, de-flooded engine cleaned spark plug it only stays ticking over wen using the throttle once released engine dies as using the throttle sometimes goes into high reaves and dies to does any one have any idea PLEASE HELP thanks