Series 11a with weber carb stalling

I have had a problem with the engine stalling once warm when I pull up and come to a stop. I have changed the Carb to a Weber so that is brand new, in the hope that this might help and also get better fuel economy. However I am still getting the sam problem. I think this points to another area. feels like someone cut the fuel. It takes a few goes to start afterwards as there is a lack of fuel. I wondered if it might be the pump, but was confused as to why it runs ok under acceleration and only cuts out when I come to a stop. Anyone got any ideas or seen this before?? Thanks in advance.