I hav1981 f150. I have found that I have one random vacuum line, coming from behind the carburetor and no clue where it goes. I removed the unused smog pump. But it only had one line on it and I know where that one went. rebuilt the carburetor. But I'm fairly sure I have all the lines in the correct places. maybe not though. So other than heading towards the front of the engine I have no clue where this one goes. maybe the second nipple on top of the air filter pan?

It has constant vacuum to it.

the empty nipple to the left is where the line ran coming from the smog pump.
Its black with two narrow red stripes.

 

Do you have vacuum to the air cleaner? You should, assuming you are running the stock setup.

 

Yes I do. On the left side from a dual inlet

 

Ok. You only need one source of vacuum to the air cleaner - which is a vacuum line I forgot in the other thread.

Anyway, I wouldn't get hung up on where that one is supposed to go as originally there were many vacuum line on the engine that aren't needed since you've removed many (all?) of the emissions "stuff".

The little block/manifold on the firewall should have vacuum to it from the intake manifold - probably from the "tree" that screws into the manifold near #4 cylinder. And, it should have a hose going to the transmission, assuming a C6, and a hose going to the HVAC system via a tee/check valve combo. In addition, that "tree" should have the line to brake booster and the one to the air cleaner.

Last, the vacuum advance should come off the carb on a port that doesn't have vacuum at idle.