It happens when i gas it from a stand still and when moving at relatively low speed, but not all the time. Ispoke to the dealer and he reckons it is slight flooding of the carby and said to not hold it open, apart from that he couldn't or wouldn't give me more advice, I tried opening the plug gap slightly but there was no change, i un hooked the rev limiter wire but still happens 80% of the time, thanks for your help.