The Tube that is just in front of the fuel inlet on the #2 carb is just a vent to atmosphere. On the 78s i believe they used a fuel screw just to the front of the bowl. I usually set it at about 3 turns out. I think the top screws you may be referring to is just a port to plug in a carb...
You can bypass the fuel valve. I believe the vacuum on it attaches to the #2 intake, and the valve itself sits just above the 3 and 4 carbs. It is possible to remove it and just gravity feed the carbs. I believe they actually refer to that valve as the sos "spawn of satan" on some of the dohc...
I believe those carbs have a vacuum activated valve on them so it may take alot of cranking before you even get fuel to the bowls. You'll probably want to take a look into the tank and see if its clean or rusted inside. The carbs will probably need to be cleaned if its been sitting a while...