Carb rebuild.SU HS6

[Ron Kwas]

Derek;

I guess my groovy idea was nothing new...I gurgled as you suggested, and saw plenty other SU owners had already figured out that one man's Vent is another man's Overflow...:

...but when looking around further at that (MG?) site, I was astonished at the size of the Vent hole..much bigger than I have ever seen fitted on a Volvo:

Source of both pictures: https://sites.google.com/site/fifers...floatbowl-info

...with a cross-sectional area about a third that of the fuel inlet, it seems like they could flow the entire output of the Fuel Pump out that hole and onto the engine...not just vent out a wee bit of air as the level varied, or a few molecules of fuel fumes. WOW!

This picture is of a Bowl Cap of an SU fitted to a Volvo... Vent is 1/16":


...I have to wonder why there is such a huge variation in the Vent/Overflow Hole!

Cheers

[arcturus]

I agree. It is some sort of device for controlling overflow. Whether or not my one still works is another matter.

[simonvolvo]

Originally Posted by arcturus

BTW what exactly is the function of the other part with the spring ? My guess is that it controls the balance of fuel to the carbs and is also an overflow valve.

My car (1970 122S with B20 engine) was provided with one of those as well, and still has it.

See

http://weblisher.textalk.se/gcp/2016...0&noConflict=1

and go to page 105.

It is called a “ventilation unit”.

The tubes 7 lead from the vents on the float bowl lids, and the tube 8 is connected to a fitting on the underside of the air filter housing (I have one of those rectangular plastic box housings).

As far as I can tell it connects the vents to the air filter housing when the accelerator pedal is released. Presumably this allows the engine to extract petrol fumes from the float bowls when you’re stuck in a traffic jam. I’m guessing.