The newly overhauled Villiers engine of my Geest autotruck runs very well indeed until the spark plug soots up heavily, very soon after starting – presumably because it is running too rich. I have removed the air intake choke in case that is the cause. Forgive my ignorance but can somebody advise on carburettor mixture adjustment – see picture. Presumably the adjuster is the knob and locknut on the left of the picture? Does it screw in or out to weaken the mixture?