Those aren't the bleeds. There are two adjustable air bleed screws located there. The actual bleed passage can be seen inside the choke tower--they're the outer set of holes. The air bleed screws intersect the bleed passage and were used at the factory to fine-tune the idle air/fuel mixture for the lowest emissions. The carbs which had these screws from the factory did not use a conventional pressed in "restrictor" bleed on the idle air circuit on the primary side (inside the choke tower) as most other Holleys do. They are well-known to be a potential source of problems, and many rebuilders/restorers disable them and convert the idle circuits to a fixed calibration by installing air bleed restrictors as found on conventional Holleys.
Actually, the DZ carbs do have the adjustable air bleed screws. Many GM Holleys from `66 to `70 have them. I convert every one that comes in to fixed calibration and I've not had a problem with it yet, as opposed to when I left them alone--I had a number of come-backs with complaints of getting the idle mixture to take a set. Those screws are very susceptible to vibrating out of adjustment, and naturally many novice tuners spot a previously undiscovered screw and just have to give them a turn...
Disabling them and converting to fixed idle calibration (as with "conventional" carbs) simply eliminates the variable.
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