Large RPM and Vacuum drop when shifting from park to gear

A ported advance does increase timing as you tip-in the throttle -which is exactly what you don't want.

It works far better to have the timing ALREADY advanced at idle. It gives high vacuum and a strong idle.

If that's true, why use vacuum advance at all? Well, it you don't have it, and you're starting at 20 degrees advance, you'll get pinging once you tip-in the throttle and start accelerating.

So...you advance the ignition at idle, then when you open the throttle, the timing RETARDS a little (like 10 degrees or so). Then, the mechanical advance is tuned to bring the spark advance back in as you accelerate. If you want to be at 14 degrees just off idle, it's better to start above that value and drop to it than to start below it and go up to it.

It works wonderfully. If you haven't tried it, don't say it doesn't work. Vacuum advance was used to meet emissions regs at idle. It's not what the engine wants.

You can see this if you map an EFI engine's spark curve. You pull the timing way up at idle, then let it decay as load increases and vacuum decreases. Pretty much every EFI engine ever born does it. The above method is the poor man's way of copying that.

As for mechanical advance not being able to advance as fast as you'd like....trust me, a centrifugal weight can accelerate far faster than a vacuum pod.