Isn't kick-back caused by pre-ignition, from bad plugs or advanced timing? I.e. firing inside the cylinder before the piston has reached TDC. Or, possibly not pulling quickly, hard or far enough to get past one of the compression strokes, at the time of ignition. Thus, causing the engine to backfire. Like the people who were unfortunate enough to have arms and wrists broken, apparently trying to hand crank Ford Model T's.
Anyway, I only started it for a few seconds, just to see if everything was working. At the moment, I don't have a clutch half that'll fit on the PTO because it's worn and damaged, to the extent that a key won't fit at all on one side and on the other side, it is completely loose. Someone on this site also mentioned, that I shouldn't run this mower at all in this state, until I repair the cracks and sag in the frame because I risk further damage to the rails, clutch and PTO.
