I don't think that's quite right.

The RR B Range manual TSD 702 and the Power EMER S522 - B Range Engines, gives idle at 450rpm and I think at 1K it would be pulling hard on the Ferret fluid flywheel.

The exception is Stalwarts with the designed exhaust leak caused by an intentional push-fit coupling and here "up to" 650rpm is suggested to "minimise exhaust flame."

The RR B Range engine is as smooth as oiled silk in reasonable order and correctly set but unfortunately the latter requisite often eludes people who fail to understand the twin-point distributor in the sixes and eights, with half the number of cam lobes that you would seem to need, that sequentially fires and dwell angle extends alternate halves of the engine, plus the absolute necessity to time AFTER TDC (0º - 2º) and static only; not intuitive when a fixed reference pointer isn't provided either.

R.