Bike in a Box: 2003 YZ125 rebuild

The YZ125 Project.

Being a guy who loves just to go out riding on the trails, I’ve never really saw the fun in motocross. Whenever I thought of motocross, I thought it would be boring, that it was just hitting the same jumps while going in a circle over, and over, and over again.

Then one day, I felt adventurous and threw my Husaberg and gear in the back of a truck and went to my first motocross track. I wish I could say I was a natural at it, and that I was throwing some massive whips and getting guys asking how to do that, but I can’t.  I ended up just rolling most of the jumps and was being asked what a Husaberg was. Towards the end of that night though, I had decided I was going to be brave and hit some of the smaller doubles they had there. I must admit, I wasn’t the most graceful in the air, but that didn’t deter me from being proud of myself for hitting those small jumps. The minute I came home, all I could think about was going back and trying that whole motocross thing again.

I went back to the same track a couple more times before I realized that a Husaberg may not be the best choice for a motocross bike. I was constantly having to change the suspension settings because the bike decided to seat bounce itself on the face of the jumps, causing very inappropriate words to come out from under my helmet. Then I would get home and forget that I had changed the suspension settings for motocross tracks, and would then be reminded that I did by the bike the next time I took it out for a trail ride. So It was recommended to me that I should get another bike for the tracks and keep my Husaberg for the trails. This idea excited me because it meant I could own another bike, and that can never be a bad thing.  That’s when I began to research on what bike I should get for the tracks.  After I looked around for a little bit, I decided to get a 125. I’ve heard how fun they were and I was informed that you learn a lot more on a smaller bike. Plus I felt that once I get the motocross thing down on a 125, I could do tracks on anything.

Having decided on what kind of bike I wanted to get, I looked all over craigslist for a cheap, run downed 125, I wanted to get one that needed to be rebuilt since I don’t have a lot of mechanic experience and I wanted to learn by doing. I was telling one of my friends this and he happened to have a 2003 yz125 that needed a lot of work done. Naturally, I was interested and went over to check it out. To put it lightly, it needed more than a lot of work done. The bike had been in pieces for 6 years and crank needed to be replaced, it lacked a connecting rod and top end bearings, the cylinder was junk, there wasn’t a 125 piston, only a 144 piston that came complete with a 144 cylinder that was brand new but had been ported poorly and the exhaust bridge had been welded back on, the plastics had decided to rot to the point where they were coming off in chunks just by looking at it, the frame had some rust marks on it, no seat, no pipe or silencer that was any good, and the forks and shock needed to be rebuilt as well. So I gave him 300 for it and was happy as a kid on Christmas.

The minute I got that bike in my shop, I got my tools out and began to tear the whole thing apart. I started at 10 am and didn’t leave that shop till 9pm. I had that bike 100% tore apart at the end of those 11 hours of work and I was very pleased with myself for not breaking anything. As far as the bike’s condition, passed the plastics and not running at all, I was impressed. The carb was completely clean, neither the main nor pilot jet had a spec on them, the triple clamp bearings were still good, the clutch and brake cables were still fine, and the throttle sleeve was in good shape. Having said that, there was one thing that did surprise me when I took the clutch cover off, I was looking at an empty space where the clutch should be. All that was left of the clutch was the basket, which had some bad markings all over it and just didn’t look good at all. Plus side, it saved me the time of having to take the clutch out.

Having gone over what the bike needed, I had decided to get everything the bike needed and then some. I wanted to go aftermarket everything since, in my experience, OEM tends to be more expensive and doesn’t seem to have better quality.

Starting with the exhaust, the bike needs a complete new one, I was thinking about going with the FMF fatty pipe with the FMF Shorty, since I’ve heard from multiple people that those combined make a nice power combination. Since I plan to eventually do some motocross races just for the hell of it, some added power sounds not to bad.

I had decided to keep the bike 125 because I had heard from multiple that a 144 was extremely hard to jet and really was not worth the little extra power they offer. So I looked for a 125 cylinder kit, and I found that Athena made a 125 cylinder kit that came with a complete 125cc cylinder, gaskets, a 125 piston, and a cylinder head. To save time, that sounded like the easiest option to get.

Since the bearings in the crankshaft went bad, and the connecting rod was nonexistent, I had done some research and decided to go with Wiesco’s crankshaft and connecting rod. I’ve always been happy with their products that I’ve used on my Husaberg and the price was really reasonable.

Suspension does need to be redone so I’m just going to start with redoing the seals on the forks and the bearings on the shock. If that doesn’t do anything then I’ll send the suspension out somewhere to be revalved. The springs for both the fork and shock should be good for me because the person who last rode the bike was about my weight, which is 155 lb.s, give or take a couple.

With the plastics completely rotted and thrown away, I was planning on going with the UFO plastic restyle. I have a friend who has the same kit on his yz250 and not only did I like the way it looked, the plastics looked more durable.

On top of all that, the bike needs new handlebars, grips, brake pads, chain, sprockets, and new bearings on everything. For that stuff, I was just going to try the flexx handlebars, because I’ve heard they really do help absorb the smaller bumps and when you bottom out, for levers I found stock ones in the box of goodies that came with the bike that I can make work, I am planning to get a set Emig grips for the bike as well, I had rode a bike with them before and was impressed by how easy they were to put on and adjust. For everything else, I was just going to get what was the most inexpensive since those parts are expendable.

For the rusty frame I decided to stick with the stock Yamaha blue for the color. So when I had the bike completely stripped down and naked, I had it sent out be painted and it came back exactly what I wanted. It was a beautiful Yamaha blue that sparkled in the light. It would almost be a crime to use it. Almost.

That is as far as I have gotten on the bike for now. I’ll keep updating when I get parts for it and what I am doing to the bike.