1.1 weber carb setup / vacuum pipe location?

I have a 1.1 formel E, I have always struggled to get it running right but today finally started to get somewhere. It has a new weber carb fitted, I can get it running perfectly until I put the vacuum pipe from the base plate of the weber to the bottom of the airbox. As soon as I do this and it seems to restrict the airflow and makes the idle drop and run a bit lumpy. But with it disconnected it runs the best it ever has, I should say if I block it completly it will stop running.
I have tried it on both union bits on the underside of the airbox.

The vacuum from the distributor to the other side of the weber is in connected of course.

Neither unions on the bottom of the airbox are free flowing, does that mean they could be blocked? I cant seem to find anything about them whist searching, nor what the small orange bit they go onto on the inside of the airbox is.

any help would be much appreciated! 

What type of Weber is it and also a picture of it installed would be good. Did it come with any fitting instructions at all and if you have followed them as you suggest there may be a problem.