My 14 year old son and I are rebuilding an 81 f150 for his first vehicle. He was very excited to get the motor out and off to the machine shop and failed to get a few key pictures. Now that the motor is back in we have a bent metal tube that looks to be pinched off running into the back of the carb. We have no picture of it pre-tear down and we have no leftover vacuum lines. What is and where does it hook up to? Motor is an inline 6 and the carb is a carter yfa. Any help is appreciated.

 

That is a hot air tube. The other end should connect to the top of a [heat stove]chamber on the exhaust manifold.

It is part of the thermostatic choke system, and it is very important for cold starts.

See here for a detailed description with pictures:
https://www.ford-trucks.com/forums/1...l#post15817036

 

Upper picture:

 

Thank you for the quick responses. This raises the next question, this truck has a manual choke, would this have not been stock or is the thermostatic control in addition to the manual choke control?

 

Someone converted it to manual choke, never seen one of these trucks with anything except a auto choke.

If you are going to keep the manual choke you do not need to hook that tube up, but the hole does need to be plugged. They put a calibrated vacuum leak into the choke chamber and this sucks hot air through the mystery tube. If you do not make sure it's plugged off it will suck dirty air into the choke and into the engine.

 

On my 1986 F150 with 4.9L engine, when changing to Duraspark ignition, I also changed to manual choke. Thinking that Ford designed the system to have the above mentioned calibrated vacuum leak, I thought the engine might run slightly rich if I just closed off the tubes, so I cut the metal hot air tube about 1 inch from the choke housing. I then attached a rubber vaccum line to the "stub" and routed it to the top port on the carburetor, where the clean "make-up" air originally was drawn.

I did not really notice any difference in performance, but felt that the air fuel mixture, at idle, would be closer to what was designed. It has worked well for years.

 

You are correct, there are several "on purpose" vacuum leaks sometimes designed into a engines intake system, and it will affect the idle and idle mixture when these pieces are de-commissioned.