It was being run open mouthed at 4% but I believe that is a tad excessive. When I stripped it there was a thick build up of carbon filling the depth of the squish band on one side of the piston and a corresponding build up on the piston crown. This carbon was as hard as fek and causing the piston to rock sideways. I can just get a 10mm feeler in from the bottom of the cylinder with the piston at TDC and the same with it part way down the bore. I put my cheap digital vernier on the barrel and get 70.04 at the top and 70.10 at the bottom. Piston measured 69.8 inline with the gudgeon pin, 69.78 just below the bottom ring and 69.8 at the very bottom. The ring gap is a snug 35 thou on one and a lot bigger on the other.Subsequently found brown deposits in the banjo and a fine deposit of brownish silt in the carb bowl. Petcock out and the filter had a load of brown stuff clinging to it. Tank out and on the bench I can now see it is very rusty. My question is, would rust particles be abrasive enough to cause this level of accelerated wear?
It had no apparent sign of over heating at all. The main is a 130 but we were running at 2/3rd throttle of less for most of the trip and the blug was always a bit oily when checked so tbh I struggle to see that as a significant issue hence my thoughts about the tank. 
