Well, closing the choke would pull more fuel/water mix and could get enough fuel to run faster, but now I'm kind of second guessing this.
If your idle wasn't set at 1,200 before and it's raising to that when you choke it I have to agree with the leak theory.
If it were my car I'd pull every single vac hose that was over 1/8 inch and plug it so see if anything stands out (Like smoothing out and idle lowering back to under 1,200)
Your brakes work the same?, as you have vac assist power brakes.
Does plugging the PCV change anything.
If everything above check out ok I would have to think there is a vac leak under or around the base of the carb next.