I recently bought my first bike, a 2000 KX250. Because i am a short guy, around 5'6 and 150 lbs, i loosened the nut on the rear spring almost all the way so all the tension was out of the spring to lower the bike. I noticed that when i am going full out or close to full out the rear end is really rough and vibrates quite bad. is this because of the suspension or could it be that the rear wheel was not put on right.?
because you loosened the tension on the spring wont exactly make your bike lower, it will in a way i guess when you sit on it but now your suspenion is way out of whack and is prob causing the symptoms you are explaining. Set the suspension up the correct way and your best bet is prob shaving down your seat foam so it will put you a little lower to the ground. Plus its free to do and really easy. I just did mine this weekend, although i didnt really need to I am now more comfortable on my bike with the lower seat stance
Check out this article, setting your sag will correct the changes you already made to the spring and get your rear shock back into the correct settings for your weight. Make sure you put all of the gear on that you normally wear when you ride to set the sag so it is set up for your actual riding weight. http://www.transworldmotocross.com/mx/how_to/article/0,13190,700369,00.html