Vaccum advance tap points

[Tony Bryant]

I'm attempting to work out the operation of the
vaccum advance system on my '85 Jag V12, both for
diagnostics, and a possible future crank triggered
ignition project.

This system has 3 tap points, connected to an
bewildering array of soleniod change over 
valves, vaccum regulators, delay valves, and dump valves.

Pickup point 1: Side of plenum chamber

This one I fully understand - its just straight 
manifold vaccum - which is the "inverse" of MAP 
which is basically proportional to volumetric effeciency.

Pickup point 2: Small drilling to edge of throttle plate

This one I'm a little hazy on. I assume it is effectively
a venturi tap, giving a vaccum spike when the throttle plate
is first opened a little bit. Can anybody enlighten me?

Pickup point 3: Larger drilling to just 1/2" down stream of throttle 
plate edge

When the throttle plate is closed this one must produce manifold
vaccum. When throttle plate is open about 1/3" the edge of the 
throttle plate aligns with this port. Possibly causing operation on 
the same principle as #2? Any ideas? 

'71 510 Daily driver
'72 510 Race car
'85 V12 XJ-S Garage ornament