I have a 1971 Yamaha xs1-b. I have put a little more than 7,000 miles on it since I finished it up. This happened a few days ago, never before, and not since. The engine was running, and the bike was in 1st gear, but the bike wouldn't roll backwards. I tried several times, but it wouldn't go backwards at all. It acted like something just locked up in the transmission. I drove home, and tested it at every stop sign. It worked just fine, and still is. Any ideas? Thanks.