piaggio nrg mc3 idle and fuel?

Hi I have a piaggio nrg mc3 2002. When I fill up my moped full of petrol it takes 10-15 seconds to start. But when I have half a tank of petrol it starts on the electric start straight away.
also my moped doesnt idle when I start it up it just starts and then idles out and stalls but when its warm it will idle at traffic lights its just the initisl starting it thats the problem could anyone help.