I'm still thinking about that A/F screw, and how having it all the way in didn't actually stop the scoot from running. That seems like a weird enough issue that it's worth really focusing on. So, since for your carbs the A/F screw when turned all the way in means no more AIR, right? So yeah, seems like air is getting in anyway. If you can determine how the air's getting in, maybe that will solve the problem?
I did some googling and found a few forums where someone had a similar issue—obviously these are different vehicles so grain of salt, etc.
Also, can you confirm that you have all the little fiddly bits installed correctly? E.g. tiny o-rings?