BigRedAF said:
What I'm saying is that a 125 has very little torque at low RPM's and a WR tranny would kill that bike.
The 250F is in the 125 class because of the rule that gives them more cubes to be competitive against the two strokes.
The 250F has way way more torque and does work well with a WR gear box.
The Yamaha's in the past that were used with a WR tranny were larger than a 125. They were called IT's.
The KDX is a WR style bike and they are larger than a 125 also.
Yamaha did make a WR125 enduro version of the YZ, for Australia, until 2004. I think that they also imported the IT125 to the US, at some point.
BigRedAF, one of my favorite bikes I've owned was a 1979 IT175. My '06 YZ125 (stock,) has way more power than my IT did, and when properly jetted, it has a much wider spread of power. I have been searching for a wide-ratio transmission that will fit my YZ.
Poor running and improperly jetted 125's have given them a bad reputation. Modern 125's, when jetted correctly, are incredible machines; given that they have the same displacement as your average lawnmower. 250 fourstrokes are also great bikes, but need twice the displacement to make their wide spread of power.
I highly recommend that everyone try one out sometime.