sounds like a fuel flow prob.
unscrew main jet and clean it with fresh petrol,then blow it out with compressed air. if still no go. replace main jet. while you're doing this, you should check the float and needle to make sure needle is not sticking which would restrict fuel flow. if it is replace needle as well. (both parts are cheap to buy)
re-install needle correct way up and don't overtighten the main jet when screwing it in.
