The BMW R90S motorcycle was produced from model years 1974 to 1976. The R90S Owners Club is a worldwide organization devoted to this iconic motorcycle.
There is a thriving mailing list/forum. You will need to have a yahoo login to access it, but joining yahoo is free. This is by far the best resource for your R90S questions, though this web site is attempting to become the (English language) repository for all things R90S.