piston looks ok but i little hard to tell , did you give the rings a little push too ? they shouldnt go it much really , from memory they didnt have any sealing on carby just make sure its on all the way and tighten .
but its sort of pointing to wear though , getting too hot , vaporising .
some of the later machines had a thin plastic type insulator , fitted over the inlet and carb' over that . carb could also be a little blocked that would account for running better on choke too
