Well Karl, things make a lot more sense now. The filter, which is restricting airflow, is fattening up the A/F mixture.
By the way, your carbs are of the "reverse-idle" type? Idle screws work on the air bleeds rather than idle fuel feed. The reverse system is enrichens when the screws are run in and leaned when backed out.
There are cases where a good air-cleaner assembly helps airflow, but the original Cobra intake system is not one of them. It's too bad that Shelby didn't go with a larger scoop that would accommodate a more efficient filtering system. So, we must now pay the price to keep our Cobras "original".
When you need serious power, remove the air-cleaners all together, but running with unfiltered air will cause excessive top-end wear--rings, bores and pistons.