Best thing to do there is pick yourself up a steel ball brush that goes in a drill. This is what I use on our customer's carbs when I rebuild them. It removes all the rust and debris that gets lodged around that tube in the canal, and it also cleans the threads out quite nicely. You don't want to run a tap down there when you're going to re-install that nozzle with the brass machine threads. Taps remove material, and that nozzle will be real loose going back in. The steel ball brush does an excellent job of shining up the threads and getting that nozzle canal cleaned out. I use the same ball brush to clean out the rust and debris in the inlet to the needle valve, and also the bowl drain..  HTH