3,9 EFI vacuum expected readings

Hello everybody,
long time since my last post, which means not having troubles for quite a while. However it was time for a new ignition amplifier and I took the opportunity for an extended check as a parallel problem idling refuses to be at the correct RPM to start with and I can keep it under control only by using the front screen switch trick I described somewhere else on this forum.
As in the RR tradition, all checks seem to be ok but one still sees the symptoms....! One variable I couldn't find a reference as for its expected value is the amount of vacuum in the manifold.
Would anybody please suggest a manual or a book where this is stated, or at least please report their experience to try and make a "landyzone consensus" of the vacuum readings?
Many thanks to all, as always
Adriano

Perhaps you won’t find any reference to what u required, in fact its the first time I've see any interest in the subject on engine vacuum on either the 3.5 or 3.9, so thats a fews years now

@adri1957
I had a vacuum gauge in my former lpg 3.9V8. This was over 20 years ago but I think the range should be approx 18-22"Hg.
Did a search of my files and found this from Iwema, which suggests 14-22.

Around 20" at idle is good. The key is though, it should be rock solid, not flicking about. If it is flicking about then you (generally) have exhaust valve issues or an inlet manifold leak at one cylinder.

The gauge should drop to almost zero if you snap open the throttle, then shoot up to high 20s as soon as you snap the throttle closed before settling back to c. 20 inches.

If the gauge slowly drops towards zero, or is consistently "low" then that indicates a plugged exhaust system. Consistently "high" indicates a restricted inlet system (fouled filter or that missing piece of rag from your last service )

Hi all,
many thanks for all the replies. My interest in this parameter derives exactly from what discool has reported: didn't find mention of vacuum levels anywhere in the manual, yet the book is populated with sentences such as "..a leak in the vacuum manifold..." "an air leak somewhere in the upper end could cause..." etc. so I wondered why vacuum shouldn't be measurable as much as fuel pressure or resistance at the thermistor ends, or battery voltage etc.
Now my reading is exactly as Marshall describes, 20 and solid, so if that's correct I shouldn't have leaks. Yet the engine starts at 1500-1800 RPM and sometimes, but only sometimes, gets down to 1000 (seen on the onboard tachometer ) after 30-40 seconds. If, once working temp has been reached, I force it to decrease to 700-800 with the trick described above then it stays as is for a while, but will get back to 1500-1800 if either 1)the engine is switched off and then on again, or 2)it takes a long run (15' or so) at high speed (eg. motorway) at the end of which the idle would have increased to those values. RR really have a mind of their own....
Ciao
Adriano

Sounds like you might have a problem with your idle control valve or throttle position switch.