I had my dirt bike (TTR 250) doing exactly the same thing - turned out the holes in the emulsion tube (that the needle sits in) were blocked - several of the lower ones. It would idle fine, and run OK at a low throttle setting - but give it a bit more, and it would splutter and cough - but would pick up with full throttle. Cleaned out the holes (they're real small), and it was back to good again.Got the popping figured out but now when I try and get on the throttle the bike wants to die and has no power. It idles fine. If I don't give it anymore than 1/3 throttle it is fine. Did have some fuel on the diaphragms when I took the carbs a part last time. Any insight would be appreciated.
Not saying that that IS your problem - but at least it gives you somewhere to start looking.