I ride my 03 kx 125 in the woods ALOT. I put an FMF SST pipe on it which broadens the whole power range. I had the FMF fatty but the SST is much better. The best upgrade i did to the bike was overbore it 80 over. The main reason I overbored it was because the way it was stock was very flat. I aslo had a slot put in the back transfer port that runs from the bottom of the transfer port down to about 7/16th of an inch to the bottom. I got the slot trick strait from team green off road. Once I overbored it it brought the entire motor alive and gave the bike much more bottom end. I also Have a 3.1 gallon IMS tank, 13 and 49 gearing, I also had the suspension revalved for an all around setup since I use it for Motocross harescrambles and enduros. With the over bore I tend to run a 120 rear tire and it doesnt seem to hinder it.
