2003 Yamaha YZ 125 Revival

I decided to take my YZ125 for a birthday ride back in September, since I had only rode this bike a handful of times during the year. I had a great day at Delta Raceway in Delta, Ohio (awesome track if you ever get the chance).

Made it home, washed my bike, and unfortunately I found a crack in the frame that was not visible before my day of motos. I was a little bummed as I felt this was the best I had ever ridden the YZ125, but luckily I found a 2002 YZ125 frame on eBay. The 2002 frame appears to be 95% similar to the 2003, aside from a remote mounted rear brake master cylinder reservoir and not having any circular holes around the steering neck. I assume the holes in my 2003 frame are for flex, so we will find out if the 2002 frame is more rigid.

The used frame I picked up is in good condition overall, but needs a bit of TLC. First up was straightening out the bent cable holders. They were basically smashed flat, so I was glad we were able to salvage them with a little bit of heat.

For aesthetic reasons, I had to get rid of the reservoir hose guide and mounting bracket for the 2002 remote rear brake master cylinder reservoir. I picked up a cheap angle grinder at Harbor Freight, zipped the brackets off and cleaned them up a bit more with a Dremel.

Off to powdercoat tomorrow!

Got my frame back and the frame swap is 99% complete. I went with a color called Illusion Blueberry and I’m very happy with the end result. This color matches the Yamaha blue much better. 

I did run into some issues with tolerances when re-assembling. The powder coat company I used didn’t mask off any of the areas where I honestly assumed they would. Don’t make the same mistake as me, be sure to ask them before they start. It required some hours with a Dremel and a sanding bit. Not a big deal, but definitely took some time to correct. 

Hey, I did not have any fitment issues with the motor going into the frame. It was a direct swap with no modifications, aside from the pipe mount GBR Performance fixed up for me (earlier in this thread). I did not have any issues with the swingarm pivot.

Through some research, I found that my motor is a 1996 model. With that being said, you should be able to find any motor from 1996 to 2004 and be able to swap it in.

I'll have to get back to you on the carb and electronics.

The 1996-1998 YZ125 FMF is the only pipe I have ridden with. I had bought a couple used Pro Circuit pipes with plans to restore them, but I didn’t like the fact that I couldn’t buy a new one if I would need to replace one. Thus I stuck with the FMF pipe and 2.1 silencer. 

Over the winter I decided it was time for a freshen up on the YZ125. The motor is up to 40 hours, so it was time for another top end. I've used this bike as a way to improve my skills mechanically, so I decided now was the time to keep improving those skills and do the top end myself.

Upon teardown, I found that the cylinder had previously been sleeved and it also had some gouges big enough to catch a finger nail. After doing some research, I found that the best route for performance and longevity was to find a plated cylinder. Luckily I found a really nice used cylinder on eBay. I decided to drop off the cylinder with the guys at Rydr Powersports / Turbo Speed Tuning and they helped me out by porting and vapor blasting the cylinder.

While I was in there, I figured I might as well put new clutch plates in the bike since I had a spare set sitting on the shelf. That's where things got interesting. When I took the clutch apart, I found that the old ProX clutch plates wore a groove into the pressure plate of the Hinson system. Then I found that the new ProX plates I had purchased several years ago were either manufactured incorrectly, boxed incorrectly, or just not compatible with the Hinson clutch as there was a gap between the pressure plate and the clutch discs once installed. I spoke with the customer service department at Hinson Racing and they were great to work with and helped me sort out the issue. It was easily resolved by installing a Hinson clutch pack. Lesson learned!

While I was in the motor, I installed a new set of reed petals for routine maintenance. The replacement gasket was very similar in shape but not a perfect fit, so it took some modification with a box cutter.

I took the carburetor apart to verify all of the jetting specs and found that the throttle slide had started to chip on the edge. I'm glad I caught the issue now! The OEM thorttle slide was no longer available so I did some research and with the help of some other Vital members found that Honda part 16022-KSR-734 would make for a good replacement. 

I polished the pipe for some added bling and to pass some time during the cold winter weather.

Here I got the motor re-assembled and ready to fire. I replaced any of the old hardware I touched with new bolts to match the rest of the bike.

Here is where the build stands for now. I threw on a new set of Renthal sprockets with a DID ERT3 chain. I went from a 52 tooth rear to a 50 to make the gearing longer. I also added Dunlop MX34 tires, heavy duty tubes, and a new Moose shifter. I previously had a Moose shifter and I wore the splines out. No matter how tight the bolt was, it always had some slop, but the new shifter took care of that.

I took it for the first ride this past weekend at Moto Xtreme in Hillsdale, Michigan and I was stoked on the improvement in power! I've always heard and read that you don't want a sleeved cylinder, but I didn't realize how much of a difference a plated cylinder would make until I actually got to feel it. I should have done that right from the start.

I've enjoyed building this bike and I am looking forward to riding and racing it more this year.