Quote: FWIW, I can choke the engine to die by covering both secondary bores. I can do the same on the primary side, but it takes a bit longer maybe because it is hard to seal that side.
If you can do this I'm thinking the throttle plates are not completely closed. Like Ceej said, if the plates were fully closed the engine wouldn't run ie. no air. You covering the bores is blocking that air flow so they must be open.........somehow??? 
My first thought is the secondary plates are still open a bit too far. I've had a carb were the secondary idle screw felt like it was all the way out but it wasn't and the plates were quite a bit open. I really had to swing on that screw driver to turn that screw and close the plates.
Here's a good way to check that the plates are fully closed when both primary and secondary idle screws are all the way out.
With the carb off the intake, go to a dark room, shine a torch from underneith the carb. If you can see light shining through around the edges of the throttle plates then they are still open or maybe Holley installed the wrong size plates for your carb.
If you don't see light around the plates then you can safely say the plates are shut, and if this is the case then I'm out of ideas. 
I used this method to set the secondary idle on my 390.
With your torch in place again, screw the secondary idle screw right out so you don't see any light around the throttle plates. You will still see a little at the shaft ends. Now screw in the secondary idle speed until you just see a bit of light shining through, so the plates are just cracked open. Works for me.
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