Any time the motor is inefficient it will have reduced vacuum.

I doubt that you will find many M110 engines running with 20in of vacuum at idle. To verify whether you are running efficiently the exhaust gasses should be checked and the unburned hydrocarbons evaluated.

And double check that timing. If it ain't pinging give it another 5-10 degrees and see how the vacuum looks.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician