My build is decent, but nothing like the masterpieces that Gaspipe, Colebatch, Wattner, and a lot of others detail here. I'll do my best to make it worth your time, and hope the reasons behind choosing an air cooled old-tech 250 are worth some words.
Back in January of 2010, we had a break in the weather and I went on a ride with Brad21 and the BendDualsport crew. I was riding a bike that started as an XR650L, and is now fairly modded. It's old, but surprisingly competent. On this ride, it was unusually warm, and a mix of mud, melting snow, and maybe a little ice. Trails were easy, but greasy. Looked like this:
Nothing that technical, but slicker than the pics look. I was getting smoked. It's obvious that a lighter, more modern bike would have been faster...but even guys on better bikes were having a hard time. Watching their entries and exits, changing direction and getting the power down was taking a lot of effort. Once either the front or the rear started getting away, you would lose all momentum. But one guy there, Rog, who is not the best rider in the group (sorry Rog, I'll buy you a beer..), was having an easy go of things. He was not flying, but he was going a good pace and working half as much. And it was not because he was on a better bike, it was because he was on a slower
bike. I have been on a lot of rides, and gotten smoked often, but this was a cool thing to watch.