Could be anything. Maybe something simple or it could be something really wrong. Only way to know is to take it all apart and try to figure it out. That size of carb will make great power on the dyno but it might not drive very well on the street. You'll just have to figure out what your priorities are. A nice 1050 usually works best for the street and often for the track. Once you get to the really big carbs they can act a little "funny". We've run a lot of big carbs on the dyno and most of them have problems. The 1050 design is very solid and there are really good 1250 carbs out there but the big carbs seem to have issues more often than not.

You might try Lightning Carbs https://www.facebook.com/lightningracingcarburetors/ He is a Moparts member but I haven't seen him post for a while. He has a booster flow setup so he can figure out pretty quickly if the carb is going to work or not. My guess is that your carb has some sort of booster issue. If so, it can be a difficult thing to sort out without having the correct tools. The really big carbs sometimes have booster signal issues and the average guy doesn't have a chance to solve that stuff. Good luck!