A 2-Litre jug is plenty of capacity to feed a carb. Taking off a tank and using that for gravity feed is so much work… On both of my karts you pretty much have to pull the floor pan to get the tank out.
But some things have been found out. Get a kit or two, and rebuild the fuel pump. I don’t recall reading whether you have the round or the smaller rectangular pump. Whichever, just be sure that all of the gaskets are properly aligned, and tighten the screws gradually in a crossing pattern. The flapper valves in the round pumps can be finicky… It occurs to me that if the main membrane between the fuel side and vacuum side of the pump is faulty, then that pulse line could have been scavenging fuel and air. Not good, and it could cause the runaway condition you described.
As far as the pulse or vacuum line, that is a critical connection. It is OK to use fuel line for it, but I prefer a fiberglass reinforced line, as it is very resistant to distortion due to the negative pressure.
Sounds like you have already checked out the carburetor well enough. Just be sure that the float needle is not able to stick in the inlet and block fuel flow.