does it have an auto choke? if so might want to check to make sure its working, if you take off the aircleaner and stuff, get a mate to hold the choke closed, and then try and start it.
or you could fiddle with the mixture screw, when i was playing with my carbie if i wound the idel mixture screw in to far it wouldn't start, and as i wound it out (richer) it slowly began starting and when it did start it splutter and carried on like you are describing and if i reved it it would rev fine but woudlnt idle. so yeah have a play with the idle screw as well
