Vacuum fluctuates 9-13 at idle, then steady 18 at 1300rpm

[thedillybar]

Yes I have searched & viewed these 2 good sites:
http://www.secondchancegarage.com/public/186.cfm
http://www.imperialclub.com/Repair/Fuel/VacuumGauge/index.htm

This is a 1987 Mercruiser 260, 4bbl Quadrajet. Engine has 145-160psi compression all the way around.

The vacuum fluctuates from 9-13 at idle (~700rpm) and runs pretty rough. Engine is warmed up, choke is off.
http://www.youtube.com/watch?v=xAI8soxzb50

After revving up to ~1300rpm, it runs much smoother and a steady 18 on the vacuum gauge.
http://www.youtube.com/watch?v=dFC-9eDaNl0

Any ideas?

 

[thedillybar]

Re: Vacuum fluctuates 9-13 at idle, then steady 18 at 1300rpm

Another good site
http://www.fordf150.net/howto/diagnoseengine.php

Maybe worn valve guides?

 

[thedillybar]

Re: Vacuum fluctuates 9-13 at idle, then steady 18 at 1300rpm

Idle speed & vacuum increase when I advance the timing a LOT (20-30deg BTDC). Weird.