Does the 90 XR600R have a single carb or the dual carb setup? What is involved in putting a rear dics brake on a 90 XR600R? Do I have to swap to a rear disc XR swingarm? Is the 90 drum rear brake XR600R a steel or alumiminum swing arm? What is a good jetting setup for a stock 90 XR600R with the airbox opened up? Sea level riding.