I suspect that the problem is probably related to a vacuum leak if the carb you purchased was supposed to be set up for an MG or, as others have mentioned, it may simply be too big for the engine. I once replaced the ZS carb on my 1977 MGB with a Weber from Moss Motors and it ran perfectly right out of the box, at least after I plugged a couple of now unused vacuum ports on the manifold. By the way, this was a down draft carb. The side drafts are generally too big for a stock engine and will display similar problems as what you are experiencing. By the way, my son now owns the car and is doing a conversion to the dual SUs.