No, it's not supposed to have a dead spot and that is an early incarnation of a decel valve, it richens slightly at decel to prevent afterburning bad enough to make audible backfire, which sells no cars.

It's a huge vacuum leak. I plugged mine with a random pipe thread plug that I found in my Dad's shed.

Also known as a 'Deceleration valve'. You can get them from Rock Auto.

You'll be wanting to look at power valve operation and the second barrel starting position before it opens, if dead full shut it can make a slight dead spot when it just cracks open.

I've seen engines correct lean OR rich to control decel emissions, it just seems to be the quirk of that particular motor that shows which way to go. If the engine sucks too hard on the idle circuits at decel then those adjust leaner like some Hondas do. They use a separate add-on idle air bleed that opens only at decel time. The Fords ran too lean and why the decel adds fuel there as well. Rich leads to high HC obviously, but lean does too when the combustion becomes incomplete enough. Bad enough and you get the popping noise in pipe from incomplete combustion still afterburning in the pipe. If lumpy enough instead of smooth burn it goes off all at once. Like when you used to take old car, cut the key off, pump gas 2-3 times and then turn key back on to explode the unburned stuff you just put in the pipe. We blew off a muffler on a '57 Ford wagon one night doing that. Fun.