pencer decided to spare no expense in building the coolest retro race bike he possibly could, and that meant about $25,000 out of pocket. He built on the 2004 platform because it’s the last year James Stewart won a championship on a 125, and Kawasaki made major updates to it that year, before discontinuing it in 2006. The Chevy Trucks/Pro Circuit bikes were his dream bikes.

He bought a 2004 KX125 for $1,000 (everything was seized and falling apart). The only stock items on the bike now are the frame, swingarm, seat pan, shock body, and a few parts of the engine cases; the rest has been replaced, rebuilt or completely customized. The transmission was sent to a company that recut the cogs to 7 degrees to prevent slipping gears and clean up the tranny. A new OEM crank/rod was installed and balanced and shot-peened with matched cases. Everything internal was super-finished for smooth power delivery and more oil contact; all power valve parts were DLC coated.

Allan Brown fabricated a new bump stop to fit some custom Ride Engineering triple clamps with a new offset. He then fabricated the bung and corrected the geometry to run a Showa steering stabilizer from Ride Engineering, as well as a new bracket for a Vortex ignition so we could map the bike properly on the dyno. Allan Brown also cut/lowered the subframe 5mm to help get rid of the stink-bug effect. He then made sure everything lined up and reinforced some weld areas. A company in Texas that builds superbikes for Ducati did a custom electro-less nickel process to the frame, sub-frame, and swingarm, then Justin at Motowhips put on a clear coat. The result of all of this is our largest parts list ever.

We did the test at Glen Helen where the wind was so bad that we practically had to keep wheels on the ground. That made suspension testing a little tough, but we got a good feel for the rest of the bike. The horsepower was good, but not quite a Pro Circuit race motor from that era. The sound was crisp and sharp, and throttle response and jetting were spot-on. The Kawasaki always had a stable feeling, and that didn’t change on this bike; carving sweeping lines or diving into Talladega at speed were no problem. The brakes were probably overkill for a bike this light, but I’m not complaining.

While we didn’t get to jump the bike much, the suspension worked great in the bumps and chatter at high speeds and low. The controls were all perfect and felt like a good race bike should: solid, comfortable, and smooth. This bike made the most horsepower from middle to top, which means you had to rev it to make it work—pretty standard for a 125. As fun as it was to ride, my favorite part was just appreciating all the work that went into finding the parts and ensuring that everything was perfect. Spencer did an awesome job with this project. Watch the whole test at racerxonline.com.