What are these brass tubes?

[Timothy Keith-Lucas]

See the two brass tubes coming out of my SU carbs at about 45 degrees near the throttle spindle? I can find nothing about them in all the guides to SU carbs that I have, and they don't appear in any of the pictures I've found. Left open, they suck air behind the venturi and everything goes lean. If I interconnect them with clear tubing I can see which carb is pulling harder. Any wisdom out there?

[7Westfield]

https://www.howacarworks.com/fuel-systems/adjusting-an-su-carburettor

 

crude pcv?

[ashyers]

I believe they were for the PCV system on the '73 Midget I had.

[11Budlite]

2 hours ago, ashyers said:

I believe they were for the PCV system on the '73 Midget I had.

Yes, it was part of the PCV system on my '72 Midget as well.

[IamScotticus]

Cap them?

[Timothy Keith-Lucas]

Thanks so much. I'll think about venting the crankcase and the rocker top into them to get rid of that oily air, but until then, they get caps.

[11Budlite]

Just to add some info, here's a diagram of the PCV system showing how the two brass tubes on the carbs were hooked up to the oil separator on the timing chain cover. Plus the charcoal canister and the rest of the system.

[Timothy Keith-Lucas]

And this Bud's for you, 11Budlite. Thank you.  That must have been an emissions control solution. It would never fit in my Series 2, even with the little Austin A engine. I was thinking more of just routing the oily air into the carbs.

[IamScotticus]

https://www.calverst.com/technical-info/su-carbs-connection-hif-type/

 

[Timothy Keith-Lucas]

Got it, and thanks. For the fun of it, I connected the two with a clear vinyl tube. Now I can see droplets of gasoline migrating to the left or to the right depending on which carb is pulling more. Hmmmm. I just invented a new way to equalize SU carbs....

[jbcollier]

They need to be either capped or hooked up to a "controlled" air supply.  They should not be just open.  The crankcase ventilation system works by creating a (modest) vacuum in the crankcase.  This helps with oil leaks as well.  Worn engines have drivability issues with this type of system which is why they are often disconnected and plugged.

[Timothy Keith-Lucas]

Thank you, JB.

[wdb]

Does anyone know whether pre-PVC SU carbs were jetted differently? Also I'd be curious to know which side of the butterfly that pipe exits on in the carb throat. My thought is that those carbs may work okay on a pre-PVC engine but may need rejetting. I guess I should say re-needling in the case of SU's...

[jbcollier]

They used different needles to meet emission requirements.  They do not need to be “jetted” differently though.  As long as the engine doesn’t have excessive blow-by, the crankcase ventilation system actually works quite well.