Manifold Vacuum Advance what am I not getting?

Bewy said
CFD244,
Replying to your question in post #142.
Centri curve #s. Amounts are in dist degrees, so double for crank deg.

Start: 0-2* @ 500 rpm
Intermediate: 10.5-12.5* @ 1000
Max: 13-15* @ 2200

Some motors won't like the advance curve to start below or at idle speed. They will hold a little advance when first returned to idle from a higher rpm causing the idle speed to be high. The unstable motor then slows slightly causing some centrifugal timing to drop out which in turn causes the motor to dip in rpm causing the motor to drop a little more centrifugal timing out causing the motor to drop in rpm.... Sometimes to the point of the engine dying. if you rev the motor again the whole cycle starts over. This can also be triggered by an ac clutch kicking in or an auto car when placed in gear. A motor with a good strong idle doesn't usually have this problem but some do. Sometimes it's best to start the mechanical/centrifugal advance above idle speed.
This same phenomenon can be complicated by manifold vacuum with engines with low vacuum and a vacuum canister that has a spring that is too stiff. If the vac can is not fully applied at idle and the unstable idle dips a little in rpm the vac will drop causing the timing to drop a little causing the rpm to drop a little causing the vac to drop a little causing the timing to drop a little causing the rpm to drop a little causing the vac to drop a little.... Some vac advance cans have an adjustment for the preload on the spring in the canister. These cans usually have a hex on the front by the vac port. It is adjusted with an allen wrench through the vacuum port.
I'm not saying you shouldn't use an advance curve that starts at or below idle speed or that you shouldn't use manifold vacuum on the advance can. Use what ever makes the most sense to you for your application. I'm just making you aware of what you may run into. Manifold, ported, centrifugal and programmable electronic are all just a means to an end not right or wrong. In some applications simplicity reigns as king. If that means welding the timing in a locked position then so be it. I admire people who are willing to put the time and effort into tuning their motors.

Step 1. Verify top dead center.

Edit: to make this post longer to guarantee no one will read it.