What you have circled is the vacuum source for the choke opener. This is the lower vacuum diaphragm on the right side of the carb in the picture above. The choke opener opens the choke when the engine reaches temperature. Pulling the hose off the metal tube will (1) close the choke when the engine is hot AND if the choke knob is pulled and (2) will introduce air into the intake manifold, leaning out the fuel/air mixture. Putting your finger over the metal tube will stop the vacuum leak you are creating by pulling the hose off the metal tube. I don't know why your truck won't run, though.
However, search for deceleration fuel cut. This is a solenoid valve that cuts off fuel to the slow circuit during deceleration. The failed postion of this solenoid valve is to close off the circuit. So if the solenoid valve is broken or the wire is bad, the truck won't idle. The green electrical connector to the right of your white circle is the connector for the solenoid valve. Follow the wire around the backside of the carb to find the valve. If you can provide 12V to the valve (run a jumper wire from the battery) you can force it to be on. You should feel or hear the valve (click) open (try this with the truck off first). Then try running the truck with the valve forced open to see if it helps.
Since you are not afraid to go poking around in your engine compartment you should purchase an emissions manual for the 2F. It explains how all the emissions systems work and has procedures for testing each system and each component in the system.