This are the adjustment instructions from my old German shop manual. I used it on both my carburettor M110’s and they always ran perfect.
The vacuum diaphragm pulls the screw back against the resistance of the spring. So you first have to set the speed without the vacuum line attached to get a standardised reading (2000rpm). Then set the spring preload.

If you put the car in gear you will get a temporary drop in vacuum, there is a slight delay in the carburettor reaction. Without the vacuum controlled idle the rpm will drop so much that engine will stall. M110 are a really high revving engines and bit sensitive on low rpm’s.