CB750 F2 carb venting

[archmill]

Am re-building the carbs on a "T" reg CB750 F2 supersport.
The carbs are PD41B and I recon they must be the last carbs to be fitted to the 750 SOHC.
When I stripped the carbs, there was no joining tubes between the carbs but I am guessing they are just rubber tubes from the parts microfiche. Problem is, where do they vent to ?
Looked on youtube and very similar carbs have a brass tube on carb 2 which sits directly above the venting line but mine do not have this tube (sits directly behind the fuel inlet tube on carb 2, see photo where it should be but not drilled and fitted)
What I do have is an extra venting stem on the outside of carb 1 (see photo) but it is blocked off about 10mm inside the centre hole.
The carbs should be vented, otherwise surely the venturi tubes won't work properly due to an internal vacuum.
Anyone know ?

[K2-K6]

I'm not sure they had any fitted to them, no trace of the fitment in Honda's own workshop manual that I've got.

Correct carburettor for the bike PD41B with the K7 sharing type but denoted as "A" spec. I think it was only production line designation to facilitate different jet sets between the two.

Looks like they breath just through the overflow/vent tube (going out from spigot on float bowl base)  from outside, with the interconnects just giving an linked atmosphere across the four.

If not aware of this design, the  idle mixture adjust screw is fuel not air in this carburettor. 

I know there's not very good "enthusiasm" for the PD type, my view is that they're one of the the best metering  production slide carburettor made, particularly in respect of ideal mixture plus enrichment for acceleration only when needed. 
Slide lifter on choke too for fast idle and warm up.

I've access (limited) to an original one my friend bought new if there's need to check further in practical detail about vent pipes etc.

[archmill]

Thanks K2-K6, never thought of them venting through the overflow tubes but that makes sense. I never doubted that the carbs were original as the bike is a 1979, broke in 1983 with 15 thousand miles on it, got put in a shed and never saw the light of day until 2016. Good to know about the idle mixture, cheers.