Opinions

On some of the carb forums I’ve read some pretty interesting things people have found “wrong”(not built with the correct parts) regarding the 600cfm Eddy carbs.
My first test would be to try a “known good” carb on the motor and see if it behaves any differently.
That might mean seeing about borrowing something from a friend, not necessarily getting a new(unknown) carb.

Another test would be to try an open spacer under the carb to see if it evens things out some(all cylinders accessing both sides of the carb).

I agree a compression test should be done, along with a leak down test if there are any anomalies found.