Not enough charge with lights on...

I am assuming you have a single phase 120w alternator fitted - which should be up to the job. Is the problem the alternator? Could the rectifier/zener diode or control box (whichever is fitted) be at fault. In my experience, the rectifier is the most likely suspect. I have no experience of the Sparx 210W alternator. I assume it would be 3 phase - so you would need the appropriate control box to go with it. With all the spare amperage, you could run all sorts of accessories - heated grips being a favourite at this time of year. I would give the beer cooler a miss though... Gordon.

As Paul implies - if the alternator wiring is not damaged it is not likely to be at fault. It contains nothing more than wiring, and 120 watts should be sufficient. That leaves regulator or poor wiring, or a poor connection somewhere - or the battery isn't holding enough charge to tide you over at low revs (tick over with indicators on). The magnetic rotor is conceivably at fault but thats not likely either.

If the rotor in the alternator is the original - 40 years old - loss of magnetism is a likely cause of your woes. If you could substitute a known good rotor that would be a good way to check if this is the case.

I agree with everyone here. It's most likely to be a fault - usually wiring but possibly the Zener. Last suspect would be the alternator so it could be money wasted unless you check the wiring system first.