Peter

I have experienced similar problems to what you describe. I have overcome the fault in two ways; cleaning the commutator with meths and polishing the commutator by applying some wet and dry on the end of a stick with the engine running. I guess in my case I get a film of oil on the commutator, especially as it is very worn and too small diameter to re-cut the segments.
May be worth a try before you remove the dynamo.
Good luck.

Location: Beautiful North Hampshire

I get a bit nervous about recommendations to polish with the engine running, as in the Nicholson book. Has to be done very carefully, avoiding surprises from the ignition leads, fan blades, etc. If anything gets caught can break brush holders. getting someone to hand crank is an option. If using anything other then cloth the brushes should be lifted otherwise become embedded with abrasive grit.
Dynamos can of course be driven off the car using an electric drill and a socket, and observing direction.

Bob Culver

Location: Auckland

Thank you all, I'll give it a clean up and see what happens. The fact that it has been coaxed back to life before makes me think its something like this.

Peter.

Location: Inverness