Why does scooter start but engine "dies" when throttle is opened?

I am trying to help a young relative who has an AJS Digita 50cc scooter which is about two years old. The bike starts ok but, as soon as you open the throttle (or even try to drive off) the engine just "dies." He took it back to the garage where he bought it but even they seem to be struggling to get it to run properly.

Can anyone suggest what might be wrong with it?

Thanks for your help.