[W126 Coupe] Ignition vacuum advance question/problem

[Josh Allen]

Ok all, I have a puzzle for someone.

I was testing out a timing light that I'd just picked up, and using the 
somewhat newly acquired MB repair manual for the timing specs, dug in to 
see what my timing was.

At idle, it was supposed to be 6-10* advanced, with the vacuum hose 
unplugged, and it was near, and easily corrected to be that.

The question comes from the 'vacuum connected' specifications, and my 
car's apparent inability to reach the stated timing readings.
Per the manual, at 1500 rpm, it should read 10-13* advanced, and at 3500 
rpm, it should be up to 24* advanced. 
When I revved the engine to ~1500, it made around 8-10*, but further 
increase in rpm did nothing to advance the timing.

Would this be a sign of stuck centrifugal weights, somehow not getting 
the correct vacuum signal, or something else entirely?

Anyone have any suggestions to further troubleshoot what this might be 
caused by?

-Josh Allen