I finally got the Z running without stalling!!! Now comes the bad news... The back carb is leaking out of some tube pointed towards the air cleaner. WTF could cause this?

it's not the tube that links front carb to rear is it?

I don't know much about flat-tops, but I'm wondering if the float level is too high, or the float is stuck, sending fuel out of the float bowl vent hose? (Do flat-tops even have float bowl vents?)

Check the window on the side to see if the fuel level is correct. There is a line on the window showing correct height. If it's over then your most likely overflowing. They do breathe a bit, but it seems yours is breathing to much!

That's what I was thinking so I'm gonna take off the back one again and see