If you pulled cali stuff off, and plugged the airbox hole, and only one line is going from a carb, to the petcock, that is all that there is... your friend has no clue about the carbs from what you have said here...
They don't need anything but clean air, consistant clean fuel, and correct valve clearances to operate as they are..
Sumpin other than that, and we can't diagnose over the net, just make sure all 3 unused carb nipples are correctly plugged, and not drawing in air.
This being said, you have contaminated fuel, and blockages in the primary circuits within the carbs, preventing them from drawing fuel thru the low speed circuits just off idle. Simple water condensing in the tank, or having been there all along, and once the tank is removed and shaken a bit, it migrates to those circuits, is the major issue.