Hi guys, just fitted a 34/5 carb on my 1600cc ACD engine (converted to carb) but im able to idle. Realised this (arrow showing in attached picture.Can anyone tell me more? Thx

Not sure what you mean - UNable to idle? Realised this...? is not connected?...
If so, connect it using small I.D. vacuum hose as shown in attached pic. (the connections are in correct positions as shown in this pic.) This is the very last 1600 beetle set-up, with a PICT-4 carb. It has the 4 connection points on the latest-type air cleaner. It goes to the second one from left... If this is not possible, blank it off, with small amount of hose and a nail stuck in it, or whatever.

I can't remember what that round-thing does, not much presumably. It isn't on the 34PICT-3, but is included on the 34PICT-4. Doubt it would prevent idling?
(Yours looks like a 34PICT-4.)

Note: the distributor in the pic. is a 009, so the vac. advance spigot on LHS of Carb. is dis-used & blocked off (nail in short piece of hose).

Have now checked an old carb., and that vaccum operated doo-dad has a small plunger-valve inside, with an on/off type action. It connects via a hole inside the bottom of carb with a drilling on the inside of the accelerator pump diaphragm, so must modify its action somehow.
The book however suggetsts it could be "a thermostatic device which varies the accelerator pump's injection quantity in relation to the temperature of the carb. body."