I think I opened the proverbal can of worms today ! I was re fitting the generator to the bike after a thorough overhaul and while setting the drive chain tension/play I noticed it go tight then over 1/2" of slack on turning the motor over. Could a knackered chain do this ? Generator sprocket appears to be running concentric and looks fairly new. Looks like a fair old strip down to check the crankshaft sprocket.
Just when I thought I was on the home straight