What Swarfy said but also check for carbon on the slip ring. Some dodgy soft brushes and holders on the market. I cleaned the slip ring and changed the brushes and holders for quality ones from Brightspark and it was like a different bike. (You may need to change the acorn nuts too because the threads may be different).
Jim
Do you have a link to this 'Brightspark' ?
I just keep getting electricians when I do a search.
