big al

Hi I am new to forum, so hello I was wondering if any one has had a problem with the charging system on a Dragstar, I have a problem, when I go for a run of about 1 hr or more, when I stop it will not start, it has a low battery. It has been back to the dealers 6!! times they have fitted new rectifiers new batteries. When it is in the workshop it charges with the lights on with them off, but go out with if for any length of time and I am stuck with a bike that will not start. Any help or advise would be very welcome