My 50cc moped cuts out when it is running idle?

I have a piaggio NRG and when you start it it works fine and when you rev it it doesnt cut out but if you leave it running idle with no revs it cuts out after about 3 seconds, if you ride it for about 10 minutes and then stop it will stay running idle without cutting out but it doesnt when you first start it why is this? and how can i make it stay running idle without cutting out with out having to ride it for a while?