I had a ´72 xs 650. The last made without electric start so it had a slight horsepower advantage over models made from ´73 on. I bought it used in Ottawa, Ontario Canada for $500 and drove it perfectly stock from there to Guatamala City and back. My partner had a 500 honda, we used Uhaul trucks for maybe 3000 miles of a 10,000 mile journey (snow). The Yamaha was flawless stock, just chains, plugs, fuses and oil but it had a seat like a pipe wrapped in a towel. Back in Ottawa I put a two into one collector exhaust on it and it was never the same. I drove it across Canada to Vancouver Island, a distance of roughly 2500 miles. I can´t remember the re-jetting except that it took a lot of experimenting and it wss now hard to start. I think ultimately the answer came from the manufacturer of the exhaust system. On Vancouver Island I sold it for $500. My recommendation: Leave it stock, it works really well.