Speedfight 2 206 100 wrc, wont start, HELP?

My speedfight will not work from electric start or kick start, i have changed spark plug, battery, checked carb, changed cdi box, all i seem to be getting when i kick start the bike is either loud pluttering or backfiring, if it dus eventually start its will cut straight out, this all happened when the bike cut out on me when i slowed down n now it wont go! please can you help???