I got a significant power increase out of my Son's '03 Yamaha TTR125L by just removing the screen between the airfilter and carb, and replacing the stock spark arrestor with a $20 exhaust endcap from Lee Lohman (firstname.lastname@example.org). Power felt like it went up about 20% without re-jetting, or cutting on the airbox. TTR revs much easier and according to my son sounds cool. Anybody know where to get a rear fender cheap. It seems the increase in power resulted in unexpected wheelie and cracked rear fender.