I seem to remember reading somewhere that the Griffon in a
PR.XIX is an ex-Shackleton unit. As such it has a pretty high idle rpm which needed to be decreased to enable the Spitfire to taxi at a normal speed. Because of this the idle rpm of this Griffon is a compromise between keeping the engine running and keeping the revs low enough to enable a normal taxiing pace. At this specific rpm setting the engine isn't smooth.