speedfight 1000 hel help help asap?

hi i have a speedfight 100 2002 plate no and again wen going flat out it will just stop to a halt and then it wud start straight back up but the other day it happend and wont start again the immobiliser is working fine i now that ive cleaned the carb and reeds and cleaned the piston and barrel and a new spark plug its getting a spark and is getting petrol and commpression i dont no wot else to do