I have just visited this issue as part of my repower
with my Perkins 4.108
Over the last number of years I have been trying to collect every manual that I can get my hands on relating to this engine. One thing that I find amazing is that no where in the manuals
is there a central list of critical engine data, ie. oil pressure, coolant
teperature, engine rpm, etc. I have found that I have go searching through the manuals
to dig up the various specs when I need them, a real PITA. Feeling at initial engagement of my Hurth
gear with the Morse single lever controler that the clunking of the gear upon engagement was a bit severe I went after determing and adjusting idle rpm to the factory reccomended settings. I went on the net and found two older Perkins factory manuals. The first indicated idle rpm of 520 and the second indicated 650 as the minimum.
Not releying on the elecetric tach I used a laser tach directly off the flywheel and low and behold idle was at 850rpm. Using the idle adjustment screw I set the rpm at 650 and preceeded to engage the gear
and voila success, couldn't even hear the gear engage and had to look over the transom to confirm that the prop was in fact turning.
Now to try and answer you question, I found that if I tried to set the idle to the lower setting of 520 rpm, the motor would stall and fail to start, once I got back to 650 all was good. Having a curious mind and wanting to know as much about the old Perk. as possible, I did some more reading and the idea is that you should ideally be fussing with the anti stall valve in conjunction with adjusting the the engine idle. Considering that I have solved
the hard gear engagement problem, and confirmed my rpm with a very accurate instrument I am fine and will not attemt to go any lower.
If you wish I will try and find the link to the manual that will steer you straight.