My 50cc moped cuts out when its ticking over?

I have a piaggio NRG and when you start it, it works fine when you rev it and doesnt cut out but if you leave it ticking over with no revs it cuts out after about 2 seconds, if you ride it for about 10 minutes and then stop it will stay ticking over but it doesnt when you first start it why is this? and how can i make it stay tickingover with out having to ride it for a while?