Thought Experiment: New Life to old and notoriously slow CR125

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2/2/2018 5:50 PM

When I was a teenager and not willing to spend copious amounts of time tuning my bikes, my father bought me a spanking new 2001 CR125 for Christmas. Immediately, I was a little bit disappointed. Selfish right? Well my dad tried to buy me a YZ125 that day but, the Yamaha dealership was closed. The Honda dealership was open so I ended up with a slow and hard to jet CR125. I wanted to make it rideable but, at the time I wasn't too privy to much of anything. I was forced to carry a few plugs with me whenever I went further than a few feet from my tools. So, I have been thinking about picking up an 01 CR125 and experimenting with it. Adding some modern innovation and cutting and splicing to correct the known deficiencies. So here's some potential mods and tests I have been thinking about.

1.) Straighten the intake tract out and mold a newer, better, air boot. The CR125 intake manifold has a nasty hook in it to clear the shock. It is speculated that this one of the many reasons the CR125 makes such poor low end power. By shortening the intake tract and altering the position of the front of the motor, the angle of intake can be altered. A new air boot would need to be in order and one with much more volume and better air flow characteristic. Of course the shock is a limiting factor. Ever seen an air boot that wraps around a shock like a donut? Probably not....I wonder why? Maybe it doesn't work....maybe it does?

2.) Mirror the case volume and porting of a YZ125 a la Kawasaki. The KX125 got much better when they copied the YZ125...as did the CR125. The port layout on the 04' + CR125 is very YZ125esque.

3.) Swap to the Mikuni TMX 38 radial slide. The 36 TMX on the 01 CR125 was hard to jet and poorly jetted off the floor. A 38 radial slide would be a slight improvement in ease and a major improvement up top. Same carb as the 04'+ CR125.

4.) This would be challenging but, I'd like to see auxiliary exhaust ports (KTM, Suzi) controlled by the power valve added to the cylinder. New PV flaps would be required most certainly.

5.) Build a new pipe and ignition to mesh with the mods.

Of course, I'd need to do a lot of math to support my conjectures and there's plenty of other factors that go into it. All these mods could make matters worse. It's sure sounds like a fun experiment though and it would be neat to make some dyno runs and see how things worked out. I'll have to keep my eye out for a beater CR125.

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2/2/2018 6:15 PM

You could just pick up a YZ 125 and save yourself a lot of grief ....

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2/2/2018 8:25 PM

sandman768 wrote:

You could just pick up a YZ 125 and save yourself a lot of grief ....

Too easy lol

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2/3/2018 12:07 PM

Get the husky and your screen name will still match.

I still can't figure out the nostalgia of those old cr125s that were slow in 2000.... but to each his own, smile

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2/3/2018 1:00 PM

1) find a CR500 engine
2) install
3) smile as you vaporize every berm you come across

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2/5/2018 10:52 AM

Not an '01 but on an '02 -

HPP Cylinder
05 Air boot (probably won't work on 01)
38mm Kehien Air stryker
5mm exhaust spacer
03 KX125 OEM pipe


Not giving anything up to YZ's from riding them back to back..............

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2/5/2018 3:00 PM

99 power valve
Respond to big bore well I've read in regards to crankcase volumes of the Cr.

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2/5/2018 3:07 PM

Tracktor wrote:

Not an '01 but on an '02 -

HPP Cylinder
05 Air boot (probably won't work on 01)
38mm Kehien Air stryker
5mm exhaust spacer
03 KX125 OEM pipe


Not giving anything up to YZ's from riding them back to back..............

Interesting...was the YZ in good shape?

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2/5/2018 3:10 PM

Jrewing wrote:

99 power valve
Respond to big bore well I've read in regards to crankcase volumes of the Cr.

I don't like the big bore route because it was not AMA legal. I was always curious to know why HRC never did anything obvious to correct the supposed issues. I would have hacked and cut that motor up so much you wouldn't be able to tell it was a Honda in the first place.

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2/5/2018 4:09 PM

Tracktor wrote:

Not an '01 but on an '02 -

HPP Cylinder
05 Air boot (probably won't work on 01)
38mm Kehien Air stryker
5mm exhaust spacer
03 KX125 OEM pipe


Not giving anything up to YZ's from riding them back to back..............

sdhuskyrider wrote:

Interesting...was the YZ in good shape?

My kid has ridden multiple 04+ YZ's and 16-17 Huskys. We had a friends '05 left at our house for a year. He never rode it. I offered to buy him a new Husky he declined. The Cr isn't as smooth or refined as a newer bike but motor wise and B-kit suspension make it close enough. Plus, everyone thinks they are slow.
I rode it for the first time in a long time a couple weeks ago and was having almost an easier time clearing stuff than on my '17 250f. Like I said, done right they rip.........

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2/5/2018 4:11 PM

Jrewing wrote:

99 power valve
Respond to big bore well I've read in regards to crankcase volumes of the Cr.

Can't run the HPP PV's in the 00+ cylinders. Need to go with whole thing which is why the '02 Cr125 is the unicorn. Gen 3 chassis with HPP compatible bottom end..........

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2/5/2018 5:50 PM
Edited Date/Time: 2/5/2018 5:59 PM

2.) Mirror the case volume and porting of a YZ125 a la Kawasaki. The KX125 got much better when they copied the YZ125...as did the CR125. The port layout on the 04' + CR125 is very YZ125esque.


If you have the time and the $$$ go for it! The 04' CR125 engine isn't quite as bad as internet folklore depicts, but the '05-'07 CR125 engine is a lot better than the "orphan" '04 unit.

Your conception/thoughts I'd think were best left to those of us in a winter rut awaiting spring... I order all sorts of crazy/useless shit when it is cold. I blow a ton of cash on '05-'07 CR125's because I like them, pushing Into new YZ125 price territory on full rebuilds. Running a fully hand-restored Honda CR125R feels good!

I don't give a fark what other folks say. My $$$. If you feel the same way about your effort, no need to explain.

Nonetheless, chase your dream and keep us posted!

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2/5/2018 7:31 PM

Jrewing wrote:

99 power valve
Respond to big bore well I've read in regards to crankcase volumes of the Cr.

Tracktor wrote:

Can't run the HPP PV's in the 00+ cylinders. Need to go with whole thing which is why the '02 Cr125 is the unicorn. Gen 3 chassis with HPP compatible bottom end..........

99 power valve in hpp

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2/6/2018 11:24 AM

Might as well cast and machine your own engine.

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2/6/2018 6:01 PM

I can tell you this, my buddies had 01s and they sent the motors to PR2. Those motors came back the absolute baddest 125s I've ever ridden. No idea what they did but they damn sure had those things figured out.

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2/9/2018 4:32 PM

The 04 cr125 was a turd in the motor department....

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