Why not just buy a Honda XR250? I guess I could have just went out and bought a XR250 or Kawasaki KDX 200 and lived happy ever after. I have owned a XR250 and I loved the smooth power, but it had just too much heft. I thought about one of the newer CRF 250 racing thumpers, but I was worried about reliability and the cost. The KDX 200 is nice too, but I hate green.
My goal is to have a trail bike that weighs under 240 pounds full of gas; will get 150 miles out of a tank of gasoline; has smooth suspension with good handling; and at least as much power as my old XR250. Total package cost less than $1,200.
I started with a 1993 YZ 125 I bought locally for $400. It was a stock bike that has never been rebuilt or raced. It was a kid's play bike on the farm. The YZ 125 is known for having more low end power than any 125cc MX bike. The suspension is pretty good and the bike weighs 209 pounds dry. The big saddle is another plus as the 1994 and newer YZs changed to the 30 minute seats as did the rest of the industry. This is a good starting point for a trail bike. Below is what I did to make it right for trails.
Top End Rebuild. It didn't really need it, but I wanted it to be reliable. I used a standard bore Wiseco piston kit and OEM top end gasket set $110
Changed the fork oil. I didn't know if it needed it or not, but it has never been changed. I used Type-F ATF. $8. I also dropped the fork tubes as low as they would go to increase high speed stability and reduce headshake.
Changed gearbox oil. The old stuff smelled like fish. $4
Installed 12T countershaft sprocket. This will help offset the huge rear tire I plan to install. $18. The rear sprocket was in great shape.
New RK 520 Standard Chain. The O-ring chains suck the power out of a 125, so I went with RK standard. $23
Replaced stock muffler with FMF Turbinecore Universal spark arrestor for $110. I needed a spark arrestor to ride in the forest. This muffler killed some of the top end power, but helped the low end. I dropped the needle in the carb and went to a smaller main jet. I was thinking about replacing the 38mm carb with a 32mm, but the jetting changes made the 38mm work just fine with no bogging or flat spots. The bike now sounds like a 250.
Replaced stock Reeds with Boyesen Super Stock Reeds. This really helped throttle response in the low and mid RPM ranges. $36
Installed C755 Rear Tire. The Cheng Shing C755 110/90-19 tire is perfect for trail riding. This size is for a 250. I wanted traction, not controlled spin for the MX track. At around $55, it is cheap.
Added 9oz flywheel weight. This is the best mod for a 125. I ordered it from Stealthy. Make sure to also get the ignition cover spacer. With the flywheel weight, it is very difficult to stall the engine. Rear wheel braking is greatly improved. You can now putt-putt around in the woods like a trials rider. $110
Added 3.4 Gallon Gas Tank. Replaced the stock tank with an IMS 3.4 gallon. Now I have a range of over 180 miles. $240. This tank also fits the YZ250, so you may find a an used one cheaper. The stock tank is good for 115 miles if you want to keep using it.
Done! I spent about $1,100 and I have what I think is the perfect trail bike. I can putt around in the woods and rocks better than a KDX 200, and with the strong mid range I can out climb a XR250. I swear, the engine feels like a 200cc enduro bike. The handling is so sweet, plus I have all the power and traction I need with decent fuel economy. The bike is so mellow, my 13 year old daughter can ride it. My next step is to add a skid plate and lighting kit.
These mods can be used with about any motocross bike. Many experts say it is better to soup-up a trail bike rather than detune a motocross bike. This is bull, as the motocross bike has the good handling and suspension. The proof is the YZ 250 that was the USA enduro champion several years in a row. Many of the mods used for the YZ 250 were the same as done to the YZ 125 above. Do you have a old beat-up MX bike kicking around? You can make one heck of a trail bike out of it. I have this old 1987 CR500 in the barn................