Those upper vent tubes on our carbs... why are they there?

[deltarider]

Never quite understood why those upper vent tubes are there. Yes, they vent, but so do the overflow tubes.

[K2-K6]

The overflow is specifically to allow excessive fuel, from leaking intake fail, to bleed out safely (well from not depositing into engine etc) away from other parts of the bike unimpeded.

You wouldn't want the carbs to suck air from that tube ideally, possibly water etc has access.

Vent tubes allow "safe" ingress of air from discreet source, in or out, to make sure there's no pressure plus or negative allowed to accumulate in float bowl, making them as near neutral as possible so that the vacuum through the carb venturi and pulled through the jets is absolutely as repeatable as possible to give consistency in jetting/mixture.

[McCabe-Thiele (Ted)]

Interesting explanation k2-K6 makes total sense when you put it so succinctly.

[deltarider]

Where should their open ends ideally be? Saw a video recently where the owner of a CB500 had lead them between the airfiltercase and the battery. See: 10:16 https://www.youtube.com/watch?v=lkxVVq_o8mY

[K2-K6]

Where should their open ends ideally be? Saw a video recently where the owner of a CB500 had lead them between the airfiltercase and the battery. See: 10:16 https://www.youtube.com/watch?v=lkxVVq_o8mY

Likely anywhere that gives a air supply without risk, something near the airbox intake itself mid bike and away from rain etc  looks most sensible.

You wouldn't want them to be influenced by air intake vacuum though, so not directly in the intake mouth for example.

[deltarider]

AFIK mine have been dangling in more or less the same spot where the exits of the four overflow tubes are. Hence my question.

[Erny]

mine goes back to top of the battery area

[deltarider]

Please read: http://forums.sohc4.net/index.php/topic,205679.0.html

[deltarider]

Come on guys! Who has a bank of CB500 carbs on the bench and can verify this situation?

[McCabe-Thiele (Ted)]

Does this help?
#.trashed-1699798592-PXL_20230917_174014528 by Macabe Thiele, on Flickr

#.trashed-1699798575-PXL_20230917_174035269.ACTION_PAN by Macabe Thiele, on Flickr

[Nurse Julie]

They go up over the plenum chamber and hang down.

[deltarider]

Thank you for the pics, but I have discovered something very strange. Please read: http://forums.sohc4.net/index.php/topic,205679.0.html

[taysidedragon]

I read all the posts on both forums. I think you've discovered how the float chambers are connected now.
To answer your original question of "why are the 2 vent tubes there?" my theory is that they provide a secondary source of air vent. If the carbs are overflowing then the overflow tubes are full of liquid fuel and no air can pass through them. Air can still pass through the vent tubes though.

[K2-K6]

My understanding of the tubes and their need to be there in the design is for mitigation in "anti-syphon" function.

I may he wrong on this, but I don't see the aeration/emulsion performance drawing air from float bowl (as mentioned on US site) in carburettors generally.
This is usually discreet in that air to float bowl is purely supplied to maintain as close to atmospheric pressure on the fuel content only, this to give the supply effect when vacuum is imparted on the jets by depression through the venturi. in reality, the air volume in a float chamber doesn't get changed much at all during use .... the outflow through the jets is exceeded by the capacity of flow from the main fuel tank (modulated by float valve) to always replace what is being used immediately. The air in reality is vented to stop this fuel flow action be impeded by artificial pressure shift, it's literally "shuffling" back and forth (the air that is) to accommodate any system lag in supply and use. The air in there is not being consumed.

The emulsifying air is, as I understand it, a separate supply route and usually "tapped" from entrance side of carb slide areas  .... ultimately to be modulated by it's own jet or adjustment of air only to give mixture option in setup.
The fuel jets always being intended to have their entrances below the fuel level (in correct operation) would appear to vindicate this.

Its also not true that cars with common manifold over multi cylinders use CV type carbs exclusively because of this.  Many manufacturers have produced their design without using CV type carburation.

[Oddjob]

Fairly sure I remember a post by Mark Paris saying the vent tubes work best if positioned in the still air near the filter housing. So up, over the plenum chamber and down each side of the filter housing. Maybe tape them in position so they can’t fall down over time.