I'm sure I could play this game with you guys all day. There's a lot of stuff under the hood I know nothing about. IF I can locate the answer in the service manual I have, I don't worry about it. When it looks obviously bubba-rigged, I will probably ask on here. Case in point:

1974 L48 Coupe, Automatic.

Below is a photo of the passenger side of my Q-Jet carburetor. Right below the choke, in the lower right of the photo, notice the hose exiting the base of the carb which appears to have had a hex head bolt screwed into the end of it. What is the purpose of this? What did it used to do? Should it be doing something now that it isn't? Thanks for the help.

 

It looks like they plugged a vacuum line.

 

That is the vacuum line for the evap canister. It's of no consequence (other than looking Bubba, which you can solve with a vacuum line plug) having it plugged other than your evap isn't functional, which can stink up the garage a bit with gas fumes.

 

The 1974 is different in a number of ways, much like a 1968. I had some minor but annoying runability problems last summer that I "sort of" solved but things were not quite right. An off-idle stumble for example annoyed me to h***.

I was in correspondence with Lars last winter (with pics) and he noted that I was using ported vacuum for the distributor advance. My reaction was that that is the way GM did things in those days for emissions reasons. Well, no, not in 1974. Because of the transmission controlled spark (TCS, I removed mine) setup in fact GM used manifold vacuum for the distributor advance, unlike other years. So I rejigged things last month (much tweaking of idle etc.) and now the car runs much better. And the off-idle stumble is gone. It is tempting to blame the accelerator pump, and I did initially, but other factors can come into play.

The port shown in your pic is manifold vacuum and that is where the hose from the distributor should be located. Just in front of that is a snout projecting out about 45*. That is ported vacuum for the EGR controlled by a thermal vacuum switch (TVS) located on the thermostat housing. That is the only port you should use for the EGR on a 1974. The AIM shows this. You do not want EGR function before a certain operating temp (~120*F at the TVS I believe) and you do not want it at idle (manifold vacuum).

So the answer to the question is: manifold vacuum for the distributor.

 

Darn, You beat me to it!

 

My 68 L36 uses this port for vacuum advance. Much better then the timed port on this car.

 

I fell into an old trap: what is below the throttle plate is manifold vacuum and what is above is ported. I believed that for years and configured my rebuilt carb last year accordingly. Well, that was wrong as Lars pointed out in the case of a #70442xx (1974) carb. It's not what things look like on the outside but where things exit/function on the inside. It was a learning experience for me for which I thank Lars. I took the 1974 out last Sunday and it hummsss, even with points and condensor.

 

Ok so wait... my vacuum advance hose currently goes from the vacuum advance around to the front of the carb and plugs into the port (shown in the double-carb photo above) directly to the right of the fuel filter connection. Why is it currently routed all the way over there if it should be plugged into the "bubba" plugged hole?