Lots of discussion on here about exciting (?) dynamos. It is worth remembering that Lucas dynamos, whilst adequate, were not built to close tolerances. The fact that the whole thing is held together with two, very long, flexible 2BA screws isn't a good start, given the working environment, vibration and strain. Dynamos, invariably come loose and start 'chattering in use. The two useless little pegs at each end get worn, and the holes in the alloy end plates wear so the pegs don't locate properly, so the armature runs crooked in the body and makes physical contact with the pole shoe or armature. Brushes get grubby. Field coils and armatures are pretty crude. The proximity of the field coil to the pole shoe varies depending on what time of day, particularly Fridays, Lucas made the field coil. The yoke (body) is inclined to go oval if held too tightly in a vice or clamped too tightly on the bike, which will affect the proximity of the armature to the pole shoe and can cause physical contact. Another thing which affects the proximity of the pole shoe to armature is the extent to which the armature has been skimmed at time of build, also the quality of the pole shoe casting, which is variable. The proximity of armature to pole shoe is absolutely critical in terms of initial excitement. The commutator end of the armature shaft is so thin it can be bent when removing the tiny, unsuitable, bearing at that end, consequently the armature won't run true etc. etc. The Tufnol brush holder plate is held on by three pathetic little brass screws (5BA - if I remember correctly) which allow the plate to flex thereby affecting the quality of contact between the brushes and the armature. All these things will affect the ability of the dynamo to get excited. So, given the dreadful build quality, don't expect consistency. I spend a lot of time fixing dynamos and it's not an exaggeration to say that, in terms of condition, I haven't come across two that are the same! They're all worn to a degree. In spite of all the above I love Lucas dynamos because they're so simple to work on and it is possible to transform all the crap that gets sent to me into reliable bits of kit if they are put together properly!