Yes, that's what vacuum does. And exactly why most setups hate to run on full manifold vacuum.
As you open the throttle, vacuum signals naturally drop. Sometimes precipitously. The faster and farther you open, the lower it drops, until things equalize and your engine can pull more vacuum again.
Under very little load, such as level ground or just siting there, your vacuum will be relatively high. Under more load, such as going up a hill or towing a heavy load, the throttle blades are open farther and vacuum is lower.
The same theory holds true for ported vacuum of course, but it's modified by the port's location in the throttle bores. At low idle, there is zero vacuum. This is where you set the base timing. Just as you crack the throttle, instead of losing signal from it's max, you actually gain signal from zero to whatever. Not going to be full vacuum usually, but the initial spike is still fairly high and helps you accelerate and avoid bogging.
You still lose some signal as you push harder on the throttle, but that's perfect, as you want the ignition timing to fall back a bit from it's max vacuum-assist point, to avoid pinging.
Best thing for you would be to figure out why that port has full vacuum signal, or find another port that bleeds in from higher up in the throttle bore.
If you're able to remove the carb, you can trace where each port gets it's signal. Anything below the blades is full manifold vacuum all the time. Anything above them at idle is "ported".
Short of removing the carb, at least peer down the bores with a flashlight and look into each bore to see if you can see a slot (it's a transition slot you're looking for, rather than a single hole) and find out where it is in relation to the blades.
And verify too, that, with the choke all the way open, and the idle speed screw unscrewed to the point it's not touching the throttle lever, your throttle blades are closing literally all the way down so that there is zero gap on either side of the blade and the bore.
Something could be hanging them up and you're basing your idle on a falsely high setting.
Paul
