[Mike_The_Grad]

With those black nitrophil floats you dont submerge them in water to find a leak. That only works on the brass soldered type floats. The way to check the black foam type of floats is to remove it from the carb and let it sit on a paper towel until it dries out. Really depends on the fuel evaporating. Then you can either still see where fuel is wicking out of any cracks or dunk it quickly into some fuel and pay attention to it evaporating. It should look like its evaporating somewhat evenly and not like there is a crack with fuel evaporating more slowly than the surrounding area. Definitely check where the metal arms enter the foam. It may look just fine when its dry but it will show any voids in the foam by way of evaporation. Dont use anything other than fuel to check for leaks. Brake cleaner, carb cleaner, starter fluid, water, acetone, etc. Will all either degrade the foam or soak in and take forever to evaporate if at all. There is a common suggestion regarding carburetor floats. And it goes," if the carb originally came with brass floats, stick to brass floats, or if foam, stick with foam. Dont change from one to the other. Also, try and source american made BRASS floats. As the import brass floats seem to have issues with the soldering process." I went through 3 different brass fuel sender floats before I got fed up with the vendor sending me more import floats that I switched to a plastic float that is near identical to the brass one. So far it works.

Your vacuum canister is definitely a vacuum leak and is why you see no change in timing when you tried sucking/blowing through it.

The purpose of using vacuum gauge while adjusting your idle/air screw is to attain the highest possible vacuum at the lowest acceptable idle. Your results reflect this. A rise in idle rpms will increase vacuum up to a certain extent. I like your reading of 1.5 turns out with 680 RPMs and 21.5" Hg. That is damn near perfect as you're gonna get. Tells me you have no issues with piston rings, head gasket, intake gasket, valve guides, or base timing. All good things.
BUT.
You have a vacuum leak. Just not at idle. Because the vacuum canister is hooked up to a ported vacuum source. Meaning it is a vacuum source above the throttle blades. Which is why you can idle it, but as soon as you give it gas, you open up a vacuum leak along with a malfunctioning vacuum assisted advance of your ignition timing. Compounding issues which may or may not be your only issues. Hopefully it is because your vacuum gauge readings seem good. The other thing about vacuum gauge readings is the steadiness of the needle during the reading of the state of the needle. Ideally it should be like the arm of a clock and not wander at all. Some slight movement is acceptable(within 1"-2"Hg.) But any erratic or continuous behavior is not.

My suggestion is to replace the vac canister and check everything again. I mean EVERYTHING. Timing, dwell, idle mixture screw, fast idle screw and vacuum gauge readings. My guess is that something is gonna change from your current setup. Those numbers I quoted up there are what you want to shoot for.
Remember, ADJUST timing with the vac can disconnected and RPMs below 750. But CHECK your timing with it both connected and disconnected.

Adjust your carb and rpms with it CONNECTED.

Good luck, I think your close to fixing your problem with the can. Let us know how it goes.