I had one last year with an Enfield motorbike. The owner had pulled the barrel and piston out because he said it was overheating. I couldn't find anything wrong, piston,rings,barrel all looked fine. I put it back together and I pulled the carby apart to check it before I put it on and the main jet was sitting in the bottom of the bowl. Put it back together and it ran perfectly. Bike had been sitting in bits for well over 12 months because he didn't know what to do with it.
