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.

[scottly]

The K7 PD carbs have teeny tiny vent holes in the carb body, with no hoses attached. The overflow tubes are not sufficient, and, if the vent holes are plugged, can cause issues from road draft and rain coming off the rear tire, if positioned directly in front of the tire. Plugged vent holes will also prevent the clear tube method of checking fuel levels in the float bowls from working properly.
The holes are only .014", and are hard to find if you don't know where to look. I recommend drilling them out to a larger size. [ Guests cannot view attachments ] [ Guests cannot view attachments ] [ Guests cannot view attachments ] [ Guests cannot view attachments ]

[archmill]

Thanks Scottly, my carbs definitely do not have those vent holes. What I think I will do is fit a "T" hose joiner between carbs 2 and 3 and run a 4mm ID hose vertical to the top of the carbs, just for peace of mind.

[K2-K6]

Good detail scottly, are those the "42" carbs specified for the F3 / K8 model designation  ? As we didn't get that iteration over here.

My feelings on float bowl vent and effectiveness:- the carb in reality should take zero air from float chamber, only fuel through the jets and with correct float /fuel height.

The need for free air is in absorbing the dynamic difference between draw (through jet use) and ability of the float valve to refill that volume promptly as it happens. In reality, very small huff-n-puff volume that's easily contained within existing equipment.

If the fuel tank breather were to be compromised , then it will try and draw air through float bowl route back up the line to "replenish" main tank, which would likely overwhelm the system and give fault.

I'd not forsee a problem of original (none of the F2 I was involved with when contemporary indicating an issue) specification if all parts were clear and functioning.

[scottly]

I *think* the K7 carbs are PD41A? If the upper vents are blocked, it WILL cause issues.
This thread may be about the same carbs as Arch's?:
https://forums.sohc4.net/index.php/topic,181930.0.html

[archmill]

Scottly, had a look at the thread and yes, those are the same vents as my carbs. the parts microfiche does show that they are linked together from carb to carb with tubes and the whole sysetem, as K2-K6 pointed out, vents through all 4 overflow tubes to the bottom of each carb.

[scottly]

The bowl overflows and the bowl vents are 2 different things. Here is another thread on the topic. (The pictures were lost unfortunately.)
https://forums.sohc4.net/index.php/topic,195195.0.html