Engine dies when throttle twisted

I've a two year old scooter it's been well looked after servicing etc but now have a problem. The scooter starts fine and idles fine but when I twist the throttle the rear wheel doesn't move and the engine cuts out? Any ideas?

 

Air leak maybe?

 

What scooter is it ??

 

It's a sinnis

 

I had the exact same issue. Turned out to be the carb.
Give it a clean out