Lonestar, I had the exact same problem with a Lucas Dynamo I rebuilt 15 plus years ago. The armature and field coil all checked out electrically and it would run like an electric motor but would not charge until spun ata lot higher RPM than usual. The problem was the air gap between the armature and the field pole was too large and so the residual magnetism struggled to overcome the air gap as the efficiency of the magnetism drops off rapidly as the air gap increases even slightly.
As I had a box full of Lucas Dynamo spare parts I just used a different body which cured the problem but for yours it would be interesting to see if the new armature measures slightly smaller diameter than the old armature. Just a few thou smaller difference can be enough to cause the problem.
As I had a box full of Lucas Dynamo spare parts I just used a different body which cured the problem but for yours it would be interesting to see if the new armature measures slightly smaller diameter than the old armature. Just a few thou smaller difference can be enough to cause the problem.
