125-200cc 125 performance mods

I’m curious what other 125 owners have done to their motors and what the results were.

What I have done, ordered from most effective to least.

Replaced the TMX with a Keihin 36mm PWK. This is the most bang for the buck. Bottom end and midrange are greatly improved. The power is very smooth and linear. The bike is much more forgiving if the jetting is off a bit. I think I lost a little top end but I don’t really notice it on the trail. I would really like to try a 38mm PWK.
Unless you run wide open all the time jetting is very important on this bike. If too lean or rich it is just too hard to keep the bike on the pipe without a lot of clutch abuse. If jetted sharp the bike will pull well on the bottom and mid and get up on the top end when needed without any problem. Since needle swaps can be done in about 3 minutes on the trail it is very easy to dial this bike in.

Turbine Core II silencer. I originally bought this just because the stock unit is such a PITA to repack. I like my bike quiet but the stock one was loud anyway once on the pipe. I was really surprised when the silencer change gave a substantial midrange boost. I had to lean the jetting a bit for best results.

Head cut by RB Designs. The stock squish is really not bad. It was 1.2mm. Since RB is right near where I live I figured it was worth $45 to get it perfect (1mm). Replacing the stock .5mm base gasket with .3mm would do the same thing but I didn’t want to mess with the port timing and I wanted the bowl reshaped to keep compression from getting too high. This mod made the midrange come on harder. It also makes the fuel burn more complete which makes jetting easier.

V Force II reads flipped to “low end” position. Very minor difference.

Timing advanced just a hair. Very minor difference, I run straight Av gas though so I knew I could get away with it.



What I haven’t done but am curious about

I was thinking of having RB Designs bore my carb to 37mm and put a divider in it. Or just get a 38mm PWK. Not sure it is worth the bother though.

Porting. When I did the top end I noticed the case and cylinder ports have virtually no mismatch so nothing needs fixed there. I don’t know if porting could do much for this motor or not. I wouldn’t want to make it a top end only motor. I have not heard any report from anyone running a FBF ported 125.

Big Bore. I considered this before I did a top end rebuild. Really the bike has enough power as is so I don’t want to mess up reliability. From what I have read a 133 still feels like a 125 and is reliable. Some say it is not enough of an improvement to justify the cost though. Eric Gorr can do a 144 which is what I will probably go with if I just decide I want more power. Uptite does a 166.

Pipes. On the older models different pipes, especially the Dynoport, are supposed to help a lot. The newer bikes have better stock pipes so I don’t know if there would be much improvement or not. I will wait until the stock unit is smashed and then probably just get a FMF.

Reeds. I would like to try a Boyeson Rad Valve some day (I assume the Honda CR125 would work) but don’t feel like going through the rejetting that is mandatory for it. I noticed that most of the YZ owners prefer the Boyeson over the V-Force so it might be good.

08 cylinder. I have no idea if it will bolt onto my 06 or not. I have heard a couple reports that it produces a lot more power but then others like Ajaxauto make it sound like the 06 and 08 run the same. I guess I need to spend some time looking at part numbers. I’ll probably get more excited about this option if Husky brings out a factory 144 cylinder. I am guessing this would be a real expensive modification.

Ignition – I am curious what the CR ignition does but I doubt I will every try it unless I get a ride on a CR and really like it. The FBF ignition looks cool assuming it has some good woods maps. This all sounds way too expensive for the benefit though.


Actually I am real happy with my bikes motor and consider it “done” but it’s fun to tinker. I’m curious what else gives the 125 a good boost.

Realistically, I’ll probably look for a set of TC/TXC 50’s or FBF 50’s before I spend any money on the motor.

I just picked up a new 2006 WR125. I haven't ridden it yet but I'm so gald to have a small bore again. My last WR125 was a 2002.

I really don't know why everyone complains about the TMX. I think the biggest key that everyone overlooks is the slide. Going to a 4.0 makes all the difference with this carb on this bike. I'd have to go back to my notes to see what else I had in there, but never had any jetting issues that I keep reading about. I'm in Illinois so I'm at sea level, perhaps that's the difference. I'd only ever change the main depending on big swings in temperature. The bike would always sing.

Since it's a 125 I do like to wring them out and thus prefered the pro circuit pipe. I had the shorty silencer (R304) on there, and will probably go that way again for the 06. I may also try the regular 304 as it will allow the bike to rev further, sacraficing a little bottom. I'd like to test those back to back. This time around I'll have the power now installed and a Vforce3. I wonder if that loudmouth airfilter would make a difference? Hmmm.

The larger bores (144, 166) sound good, but I've heard reports that they are more jetting sensitive. I'd rather not have to play with the jetting that much. I'll stick with the 125 for now.

The ignition I think is personal preference. They both have advantges and disadvantages and I believe comes down to rider preference. I prefer the WR ignition, I like the added flywheel weight to smooth out the revs (if revs can be smoothed on a 125 )

I also agree that spending money on suspension and tires vs. the motor is more important and I'm looking at the FBF 50's myself.

'02 WR125....
Power Now: I think it helps the TMX work on the 125. You still need to jet for changing conditions.
DynoPort pipe: Good bottom and smooths out the transitions, mid to top.
EFM Auto Clutch: Makes the bike easy to ride and smooth.
51t rear sprocket: A little more grunt for the big hills.

I've got a brand new unridden 04 CR125 in my garage. A lingering shoulder injury has kept me off of it but I'll take it out probably next month for its first ride. I've got a bunch of needles to try out with the TMX and also have a 38mm PWK if the stock carb doesn't work out. I'll use the stock pipe/FMF twin core and probably replace it at some point with a full FMF. I'm looking forward to riding it before the Red Sticker season ends here in California.

My WR pulled a lot longer on top when it was new. I think the 36mm carb took a lot of the top end away. I think a 38mm PWK might be the best option.

I pulled out my notes from 2002, here is what I was running in the TMX (2002 WR125) back then...

400 - main
30 - pilot
6DJ8-60 - needle
3rd position - needle clip
1 1/2 turns out - air screw
4.0 - slide

100 Octane rocket race fuel
50:1 full synthetic spectro premix

I'll have to see what the 06 is like once it warms up around here.

I don't have my notes for my 2006 CR 144. There were to many changes, it was a bitch to jet! I had an Eric Gorr 144 kit and motor that was modified to the max. Best bike I have ever owned until I bought my stock 08 CR.

Here is what I ran all year here in Illinois and the bike ran just as good as my 06 144.

TMX carb
power now
3.0 slide
6DJ8-61 needle
clip in 4th pos. from top
390 main
40 pilot
1 1/2 turns out @ 75 deg. (adjust to temp)

100 octane Sunoco (street legal)
Spectro synthetic premix 50:1

FMF Fatty Pipe with Power Core 2 Silencer

Tight woods I ran 12:50 gearing and open courses 13:50.

Cool, Thanks for those updates. Not too far off. I'll have to see where the 06 lands in comparison to these. The 08 has the updated cylinder which probably required a few tweaks.

Are you considering turing your 08 into a 144? What was the biggest jetting problem on your 06?

And back to the original post, gearing as you mention can have a decent impact on performance depending on the terrain. Back in 02 I'm pretty sure I was running a 13:52.

How far (racing) could you go on the stock tank?

Assuming the bike is warm and you run it a little at high rpm to kind of clean it out it should not do that. Mine only does that if lean on the needle.

125

there are a boat load of mod's for the 125 on TT. spacers for the reed blocks, jetting tips, pipe suggestions etc.

In my 07 WR, I'm running the FBF GP piston(4 lubrication holes vs. 2), 0.3mm base gasket, 38 PWK w/ RB divider plate and air screw, VF3 reeds, FMF fatty/turbinecore 2 at 32:1 Yamalube 2R and pump premium(91-93).

All these amount to a very versatile package, the little engine will literally lug up hills and still get up and go when you want it to. With the RB carb I feel the needle is more touchy, but generally it's easier to change a clip than both the pilot and main.

It's hard for me to say which was the most bang for buck, cause a lot of these mods went on at the same time, due to engine problems and the crappy VF2's dying on me for the 3rd time.

I can say that friends who have ridden it before and after agree it is better...smooth but pipey when you want it to be.

I replaced the stock pipe in my 03 WR125 with a FMF. I looked at the stock pipe and it had a restricter welded in it and blocked about 1/3 of the pipe. FMF made a BIG diference and helped the jetting.
Hodaka Bob

Well you know what i am going to say STOCK Update after 4 years of racing on my 2006 and not rebuilding after winning the # 1 AMA plate in 2008 My son takes the bike and goes out and wins the AMA #1 plate for 2009 on it So we just got it back from George at uptight new bearings thru out new stock piston in the same stock cylinder same clutch basket.Everything looks goods .Rode it all this past weeken side by side with my
2008 and both bikes are about the same . I do not know why but but CR 125 seem to just work for up We do run 13/52 big difference even in the wide open desert 13/52 works the best .
Last weeken at the AMA National Hare and Hound race i was a loop captain so i pre rode the course many times .First on my 510 then on my WR 250 but then i had the most fun on the 125 .The little bike just works first 40 miles were fast then the next 40 was tight but then the last 15 was crazy with about 10 miles of first gear rocks up and down and over hills .The stock CR 125 worked the best for me

My son did some quick testing last week, my stock 08 Husky CR125 vs his 08 KTM 505 with Ohlins suspension. It was on my back property track approximately 1/2 mile long I guess, a flat and MX combo. Well he was 4 seconds a lap faster on my little CR125 (48 seconds on CR125 and 52 seconds on the 505). Also my Cr isn't set up for him as he's 30 lbs. lighter than me and only had about 10 minutes seat time before the "experiment". So if anybody has a tip on a new 08 - 09 CR 125 shoot me the info; he now wants one LOL! Modding for performance isn't necessary as you can see by ajaxauto's post and to a small extent my post but it's just something I do with all my stuff as I have a FMF on the way for the 125.