Perhaps the problem is that your dynamo is not putting out sufficient current for the bulb you have in the headlight?
Check that the dynamo is delivering a suitable voltage and try a different bulb? Often in my experience a quick clean of the dynamo brushes can be very helpful, and it only requires the removal of the band around the end of the dynamo itself.
Agree with Allan. I just went through this with my bike. Turned out that one of the brushes was stuck in the holder and the armature was very dark. Dressed the sides of the brush so it moved freely, cleaned the armature with gas soaked rag per manual and problem solved.
A common problem i have found is that the commutator grooves are getting full of residue from the brushes.
Some dynamo's that didn't work where ok after cleaning.
After that i clean the copper with some steel wool or scotch brite and clean it off with some petrol.
The person who made this bit explains it rather well in my opinion.
Starts at 8:20
https://www.youtube.com/watch?v=W8mSgOOzHGQ&t=701s
Regarding 'smearing' of the brushes on the dynamo commutator...Try to find some original Lucas brushes...Some of the pattern brushes are too soft and create the problem pretty quickly...You'll find original brushes on ebay....Ian
