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When I was testing that bike (23' YZ125x), it was pretty chilly out, lower 50's--high 40's, so I wouldn't be afraid to use a smaller pilot, start out with a 52 and possibly work your way down to a 50, depending on the air screw setting, for the warmer weather. The main may need to go down a touch also, possibly in the 180--178 range.

I get them from Jets R us. I see that they have gone up quite a bit in the last few months. This is a notched 6.0. Jets R Us Throttle valve PWK stamped 6 N502-27600 022-040 If you want to buy it under an oem part # ? It can be found on/under an 99'-00' KX125 parts fiche.

Yes, I started off using a 5.5 and then switched to a 6.0. They both worked much better than the 6.5, but at times I thought the 6.0 might be a little lean when the weather got cold/cooler, in the 50's. If it was my bike, I would just cut 5.0 slide into a 5.75 and be done with it. That 5.5 slide is out of a Sherco, and is damn expensive through them. With either slide, 5.5--6.0,I could use any oem W starting diameter needle and jet around it, pilot and main wise, with the 6.0 using slightly richer...