A mechanic's vacuum gauge is always connected to the manifold, or, a source of constant manifold vacuum. That means somewhere on the manifold of course or, somewhere under the throttle plates on a carburetor. Depending on the particular carburetor this can be tricky if you're not sure, because the connection port may actually exit below the throttle plates. As a practical matter it's going to hiss at idle when hose is pulled. That's the one you want for the gauge.

The "ported" or spark connection on the carburetor typically used for the distributor will not have any vacuum present at the port at a factory idle - 550 RPM say, because the actual opening is still open to the atmosphere above the throttle plates in the bore. If you happen to like a high idle though, it will start to pull in vacuum. In fact that's probably why people like a high idle. It does run smoother, and cooler, with lots of advance at idle. You will bust a smog test though. This is one reason though when actually tuning or adjusting a carburetor and other tasks, set the idle RPM back to factory spec or it will tend to interfere with getting it set correctly. Set it where you want RPM wise after the tuning is complete.

The reason you're seeing all that ignition advance or timing with a light at idle is because either A. Distributor is connected to a source of manifold vacuum, or B. Idle RPM is pretty high and it's starting to pull in vacuum anyway. That's how it works.

It doesn't really matter. If you like it there, you can keep it there, but I would suggest connecting the distributor to the ported connection for now. Start with everything at factory spec, according to Hoyle, verify everything works as advertised for troubleshooting purposes, and then go from there if you want later. First thing though, get everything dialed in where you know that the settings and baseline they are correct.

When setting or checking ignition timing on a "new" old engine it is wise to verify the timing marks haven't slipped off the index. Use a tool called a piston stop. Defective vibration absorbers are VERY common, the heavy outer steel inertia weight moves over the years because the rubber sandwich it is bonded to is dry rotted. SO, somebody tries to time it, and consequently it's 10°, 20° off (or more) and runs like ***. See this all the time. Sorry for the lecture but this may help someone else.

When checking or setting ignition timing it is also important that the distributor connection is disconnected and plugged, and idle is slow, factory RPM, for accurate base timing results.

A vacuum gauge hanging on a stockish, healthy engine in good condition and proper tune should indicate a steady needle of 18"-20" at a slow factory idle RPM when connected to manifold vacuum.

There are a gazillion other engine defects that can be detected with a vacuum gauge and quite a few engine and carburetor tuning techniques that a vacuum gauge will help you with, but that's the main one to look for. If you live high above sea level the corrected number will be quite a bit lower.

Usually when I hear someone talk about ignition timing puzzlers and crappy performance and/or low steady engine vacuum on the gauge at idle defective vibration absorber comes to mind.

Average manifold vacuum pulled at idle is very closely related to the base ignition timing, carburetor depends on a good strong crisp signal through the idle circuit. Low engine manifold vacuum is very often simply a matter of a combination of retarded ignition timing and misadjusted idle mixture screws.

I was going to mention the old saw "90 per cent of carburetor problems are ignition related." It is very important that the ignition system provides a fat, HOT spark, and the ignition timing is correct, before trying to seriously tune a carburetor.

If the vacuum gauge needle is erratic or vibrates it generally indicates a defect that affects only one cylinder. There are quite a few vacuum gauge charts available online, take the time to study them. It could be a bad wire (misfire) or arcing to ground, fouled plug or ignition defect of some kind for sure. Or, maybe a burned valve. What's nice about a gauge is it will tell you exactly what's wrong if you know what to look for. Might try backing up a bit and perform a compression test.

Engine starting w/ choke. That's what it's for, of course, we've gotten spoiled with fuel injection. Anytime the engine is "cold" even in the summer, at least a little choke and accelerator pumping is required. In fact, if an engine starts in the wintertime without needing the choke that actually indicates a problem, in that the carburetor is running way too rich. A carburetor/engine can be tuned pretty far off the beam and it will still seem to run OK. Fuel mileage is the tell, though.

It absolutely will, engine manifold vacuum will always be at its very highest at idle in neutral, because there is no load on the engine. This additional advance won't hurt the engine, but it is sometimes difficult to achieve a steady idle RPM. That's why "ported" distributor connections were invented, the ignition timing advance stays solid at idle.

In the 60s and 70s they found that all that additional advance, while very efficient and cool-running, contributed to smog emissions aka NOX.

Consequently they spent a lot of time and contortions detuning the engine, changing compression, camshaft duration, and other fun stuff. Clean air is important however they weren't very smart about how they went about it!

Even if distributor is conncted to a timed or ported connection it will very quickly start to pull in vacuum advance timing at anything much above factory idle and smooths idle out nicely. You can see this with a timing light when checking the vacuum advance. Keep in mind vaccum advance is completely load dependent. It's working as advertised.

Yes, it's all done by computers today. The trick then was, how to pull in more ignition advance than the mechanical weights and springs could provide when RPM is low. Using engine vacuum as a signal is pretty slick, it was used for all kinds of things including power brakes.