Vacuum never decreases while leaning out carburetor mixture

I have a Rochester Quadrajet 4MV carburetor that has 2 mixture adjustment pins in the front of the carb.

As I understand it, you tune these by continually tightening the mixture screws while watching a vacuum gauge. The vacuum will increase continually until it hits a peak value at which point it will begin to decrease. You find the point right before where it begins to decrease and that is your perfect mixture.

My question is: What if the vacuum never begins to decrease? I’ve seen it continually increase as I lean out the mixture up until a point where the vacuum will stay constant (but not decrease) all the way until I’ve bottomed out the mixture screw.

So, 2 questions:

• Can I just consider that peak value I hit as the correct mixture even though I’d be tuning to the front side of the peak as opposed to the backside of the peak (since my vacuum never decreases, I don’t know where the "backside" of this peak is).
• I would think tightening the mixture screws all the way shut would kill the engine due to cutting off the fuel supply. Am I missing something? Do the mixture screws always allow some fuel to flow even in a closed state?

Update:

I noticed that the exhaust from the boat was awfully dark. I closed both mixture screws and noticed it still is dark in color. From what I understand, this means a rich condition. The carburetor is a new-ish (reman’d) carb just installed, thus my reason for tuning it. I’m lost at this point and have concerns about running it totally "lean" (although it doesn’t seem lean). My inclination is to back off the mixture screws a 1 1/2 turns and call it good, but I’m also concerned at the thought of running it too rich, which it seems to be already. Any insight?

Update 2:

I found that the inside of the carburetor was cracked around the area where the mixture screws entered the barrel. This undoubtedly created fuel leaking into the intake that was uncontrollable.