Yz125 2006 jetting

The Mikuni carb on the 125 is sensitive to float level and the o-ring around the brass seat for the float valve can leak. If your bike occasionally drips fuel when it's on the stand, go pick up a new o-ring for that seat.

Low float level will cause the bike to break up on long high RPM WOT pulls. For me that was the hill at Washougal.

A loose ground, bad spark plug or bad spark plug boot can all cause the bike to break up. When I ran NGK 8 heat range plugs (BR8ES / BR8EG) in the colder part of the year in my 2008 YZ125, they would fail and the bike would break up really bad at higher RPM. It started up, ran smooth but went to crap under load at high RPM. Try a new plug.

Here's the jetting I run, but keep in mind that I have a VF-3 reed as well as a pipe and silencer and I run a 50/50 mix of Trick 114 leaded race fuel and ethanol free 92 octane unleaded. The oil is 32:1 Yamalube 2R, or sometimes Maxima Super M.

Screw: 2.25 turns out
Pilot: 40
Needle: 42-74
Clip: 3rd groove (count down from the top)
Main: 410
Spark Plug: BR9EG or BR9ES

I did a little moto on my 2011 YZ tonight here at my house in WA. It was 50 degrees F, 30.3 in/hg and 80% Humidity. The bike runs crisp and perfect in this condition.

In warmer weather, I sometimes change to the 37.5 pilot jet and run the air-screw in a little farther.

Always jet with a clean air-filter and a fresh or decent top end and make sure your power valve linkage isn't goofed up. The pins in the top of each valve can fall out on the YZ125.