SOHC.co.uk Forums > CB500/550

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

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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.

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