Missing a wire on the end of Tb

This blurb on bcdd CARBS says wired bcdd is for AC reasons


Emission controls and other carburetor accessories

The only part of the Hitachi carbs that qualifies as an emission control is the Boost-Controlled Deceleration Device or BCDD. There is no equivalent device or function on the Weber 32/36 DGV. The BCDD is the oval-shaped cylinder on the side of the carburetor body facing away from the valve cover. It functions as an auxiliary idle circuit that is activated by the high manifold vacuum which occurs when you decelerate with your foot off the accelerator. Under these conditions, your motor is turning much faster than it is at idle, and the idle circuit can't feed it enough mixture to fill the cylinders enough for anything approaching complete combustion to occur. The result is lots of hydrocarbon and CO emissions. This isn't particularly good for the motor either, since you are sucking oil past the rings and valve guide stem seals, and then not burning it off. Residues could bake onto to to various surfaces.

If you look into throttle bore of the Hitachi carburetor, you will see one hole that is bigger than any of the vacuum ports. That is the giant "idle" passage that the BCDD feeds the mixture through. The point of this oversize idle circuit is to feed enough mixture into the engine to allow complete combustion during deceleration. The air that is mixed with the fuel to create this extra mixture comes through various drillings in the carburetor center section. One of them has a threaded rod with a tapered point and a locknut. The tapered point sticks into a jet, where it sets the air feed.

Do not undo the locknut and rod when you take things apart. Retuning a BCDD is an unnecessary hassle. I refer you to a Factory Service Manual or Haynes Manual for diagrams and tuning instructions for the BCDD. Gaskets for the BCDD section aren't included in most carburetor rebuild kits. I just reuse the old gaskets when I take apart a BCDD to solvent soak the metal parts.

The only way a problem with the BCDD might show up is on a dynamometer emissions test which included deceleration.

The rest of the emission-control devices aren’t really parts of the carburetor. They are located in the stock air cleaner, but they still modify the mixture. There are more of these devices on carbs from 1972 and later model years, including the L18 and L20B-powered vehicles which have the temperature-controlled air cleaners. They are very good at maintaining a proper idle mixture when it is hot under the hood, and at controlling the intake of air from the manifold heat stove. You won't have trouble with carburetor icing using these air cleaners.

Another accessory is the vacuum-controlled choke opening unit, called the automatic choke unloader. It uses a small diaphragm with opposing spring to pull a linkage which opens the choke plate temporarily when you accelerate. You can check the condition of this diaphragm by compressing the rod and placing your finger over the vacuum port opening. If the rod stays in, then the diaphragm is still good. If it instantly pops back out, you need a new one as you've got a vacuum leak that could affect your idle adjustments. It is OK for the rod to slowly re-extend, but it shouldn't pop right back out. The Hitachis also have idle cut-off solenoids,an option you can order for Webers as well. Finally, some L20B engines equipped with A/C had a solenoid on the side of the BCDD to give a little boost at idle when the A/C is running.