That happened to me this weekend on my 99 WR. I was riding with a slow group in 1st gear almost the whole time. I stopped to pick a quad off a girl who got stuck under it and noticed the green stuff dripping form the overflow tank. It took about 3 mins to let all the steam out of the tanks before I could add water. Once the coolant was mixed with water it did not over heat again. I dont think these bikes like %100 anti freeze in them, and that looks like what was in it. I am going to drain the whole thing bcause I used tap water out of my camel back (thank goodness for that thing!!!!) and tap water can have miniirals in it that will wipe out the seals and eat at the aluminum. I will be going 50 / 50 water and antifreeze, maybe peak brand and I will be using some water wetter in it. This is a heat exchange catalist that makes the coolant more efficiant. I belive the YZ have smaller rads than the WR and I dont think they have over flow tanks under the left side number plate either. If it still boils over with the above mentiond coolant mixture in the slow stuff, you may want to look into some 2001 speck WR radiators for it. I belive the 01s have added cores in the rads.