Carb Issues?

At a loss to why the car is now running really rough, stumbling and hesitating. It acts like it has a massive vacuum leak but i can’t find one anywhere. I’ve tried two carbs and the result is exactly the same.

There is also a HUGE hissing sound coming from the top of the carb like its pulling in lots of air. I can reduce the sound if I put my hand over the top of the carb but it doesn’t alter the way it engine runs.

Here’s a video.