Why my 2 stoke 125 wont run?

I have been given what I think is a 125 2 stroke out of a yamaha motocrosser. We have tried to put It in a go cart, it sparks ok, and all the connections seem ok,but when we start it it revs in 2 second bursts and then dies, then revs again and just repeats. This Is with the throttle at full chat, and it dreamt run at all without throttle. Any help?