I recently purchased a 2003 DR Z400S. After riding some trails and log roads for an hour I found an area where other bikes and quads apparently travel a lot and there was a series of whoops that I found were fun to ride thru a bit faster. The first time I went across them at any speed I felt like I was backing off and on the gas. After several runs I realized the engine was surging. I was later running at some speed down hill and found the same surging was there. Being a ways from home, I thought I might not make it back. A friend who was riding with me suggested I try to drain some gas out of the carb. Initially I though that could not be the problem, but it was easy to do and try. When I loosened the screw on the carb bowl, I noticed some bubbles and what appeared to be water in the gas streaming out of the carb. It cleared up after I drained a few ounces. After I got started again I did not notice any surging and even tried to make it happen again. I could not notice any surging at all, and the bike ran fine the rest of the afternoon ride. Thought I might pass this on to save others a bad day and possible repair dollars that are not necessary should they have a similar experience.