Morris Minor Owners Club

The lower pipe on top of the float chamber lid is the overflow / breather pipe.
If twin carbs are fitted the second HS4 carb may be either served fuel by an additional pipe coming out of the top of the float chamber lid on the other side of the boss at the same height as would be for the single carb or a single pipe which splits into two to serve both carbs.
See the exploded view taken from the Burlen Fuels web site and / or the twin HS4 carbs for sale on the 'e' bay link https://www.ebay.co.uk/itm/Mgb-Gt-Pair- ... SwI8lZ8xZN

Thanks for the confirmation Phil, I will run an overflow in a piece of small bore copper pipe out of the way of the exhaust system.