Drew DellOtor carbs on Vespas have an "Idle jet" that is hard to find so often missed in the cleaning of the carb.
Look at the attcahed pic and go to the link that Cyberian posted http://pg.photos.yahoo.com/ph/bwebb08/my_photos
If your carb has this Idle jet and you clean it properly your bike should then idle OK.
You have to use a tiny piece of wire like a single strand of a control cable or piano wire. I use a very small #74 drill bit to clean the hole. I don't drill it, the bit is smaller than the Idel jet hole and is just used to clean it out,
Once you use the wire you can then use wd40, carb cleaner and then use compressed air to blow it out..
I have Carb rebuild kits for the Dell Orto carb if you need one. The kit includes the washers you need to stop the leak.
My email is zippymoped.yahoo.com