Could be that the carby float is stuck. It will only be letting through a small amount of fuel so the bike dies as soon as you use more fuel than is getting into the bowl.The best way to check it is to loosen the drain screw and see if fuel comes out. If not, take off the carby bowl, remove the float and check the operation of the needle valve. It may just be jammed in the valve seat and should be easy to release by hand.