as m says it may be the regulator but these 6 volt systems are crap and require a very good battery to work properly so put a good new battery in it and then see how it is,the lights run off the lighting coil which is ac and will get brighter when you rev it thats normal.its typical of a 6 volts bike to struggle to do a few things at once even with a good battery the indicators will not flash very well with the brake light on for instance.
adam bike doctor, June 2010