1978 B stalls immediately after starting

I have a 1978 MGB with everything original except that the catalytic converter has been removed. The car hasn't run in almost 2 years. I checked the fuel to the carb and the spark and both of those are fine. When I try to start the car, it starts and then immediately dies. If I give it gas as I start it, it will start and run for a few seconds, idling around 1100 RPMs, but then the RPMs slowly drop until it stalls. What can I do to diagnose the problem? Please note that I have barely worked on cars before, so if I have to check something, I would greatly appreciate knowing how. I have a Hayne's manual, so if it's in there I can look it up.

Thanks in advance!

EDIT: I tried starting it again and got it to run at 1500 RPMs for about 30 seconds, but then it slowly started climbing. I turned it off at 2000.