Moped Army

So I switched out my 14mm Bing and small intake (maybe a 12mm?) for a 15 Bing and 15 intake. Bike starts and runs fine, but will die if I come to a stop and won't idle. If I keep a little throttle, it will be ok. When it does die, it will start right back up again.
I've tried the idle screw all the way and all the way out and everywhere in between, same thing.
Since it was idling fine before, and all I did was switch carb/intake and upjet (from 84 to 90, now an 86), I figure it's got to be the carb. Right?
Sprayed for air leaks and found one and fixed it. Now everything's nice and airtight. Same problem.
Took off the airbox. Same thing.
Haven't tried it without the exhaust yet, but like I said, it was running fine before switching the carb so I don't think it's a clogged exhaust.
Fuel flow seems to be fine.
I didn't give the carb much of a cleaning, but it was brand new out of the box.
What else could it be?
I haven't tried switching back to the old setup, but I guess that will be my next step to isolate the carb as the problem.
Any ideas?