Well, here it is. The post you all have been looking for. If you own a TTR90, do this stuff....NOW!!! Take the snorkel out of the top of the airbox, get a TwinAir filter, install a FMF powercoreIV with spark arrestor, and put a 95 Main and 15 Pilot jet in the carb. Install some Dunlop 773's, move the clip on the rear shock and make sure you have some good fork oil in the forks. If you do these things, you will see the biggest smile on your kids faces. WOW, what a difference!!!!!! This thing runs awesome, is not too loud and can really zip through the turns. My son logged about 60-70 miles on his bike today and was passing KX60's, KX65's a TTR125L and at one point a YZ85 on the local peewee track. By the way, my son just turned 7, is about 55lbs and has been riding for 2 years. While I cannot attest to the skill levels of the other riders, he was sure giving up a bunch in the power department. His TTR has been transformed into a little 4-stroke rocket for about 200.00! Do it now.