CR 125 2003 mods

So, after being out on her a couple of times I found that she was only hitting the powerband between about 60-80% then dropping off. This had to be rectified. After some research online, there were a couple of mods that could be done.

1. Drill out the airbox to get more air in

2. Substitute the airbox intake boot from a 2005 CR 125

3. Put in a V Force reed block

4. Machine a pin that stopped the exhaust valve from closing some of the way

5. Get it ported

6. Buy some exhaust spacers

7. Change the gearing


As I wanted to keep costs down and this is information that I want others to be able to use, this was the plan

1. Drill out 6 x 1/2 inch holes on each side of the airbox

2. Couldn’t get hold of a 2005 air intake

3. Install some Boyesen power reeds

4. Machine a 7mm stop pin

5. Clean up as many of the casting flaws that we could with the tools we had and polish up the exhaust ports

6. Make some exhaust spacers which were 5mm thickness(combined)

7. The gearing already had a 52T on back as opposed to standard 51T


After stripping her down, I came across some interesting things!


The standard exhaust valve stop pin was not even there and then when I took the valve rod out, half of the bearing was missing(Which was interesting as the engine had just been rebuilt by a shop in the South West(if you want to know what the shop is, you can email me) for the owner that I had bought the bike from. He had only ridden it for 1 hour since that rebuild!)


In these pics you can see the gunk on exhaust valves


This pic shows heavy scoring on valve


Here you can see casting flaws in the exhaust port


Looking at the side of the reed block, you can see where the reeds are open



Work carried out by Ace mechanic(Dad) and myself


Various stages of transfer port work. Just cleaning up the casting flaws as porting/tuning is a science in itself


Cleaning up of exhaust valves


Cleaning up and polishing of exhaust port and manifold


Here we have the exhaust valve stop pins

The top one is the original size, which was made from a 4mm drill bit

The one below that, was made from the shaft of an old clutch lever bolt. The diameter of the stop is 7mm


Here is the modded pin(7mm)in place


These are the Boyesen power reeds

There are two layers, the top one opens up at low to mid throttle and the lower one opens up after that


These are both sides of the airbox with 1/2 inch diameter holes


The last mod was some spacers that go between exhaust and exhaust manifold. You can make a 2mm and a 3mm spacer then double them up for 5mm. We had some material that was about 1.25mm thick, so we made 4 and put them all in.

Pics coming later

Power has a lot to do with temperatures in the exhaust.

Therefore I have a theory that the paint on the exhaust helps to retain or maybe add to temperatures within pipe

So, I stripped the paint off of mine(I think it was bare to start with, but someone sprayed it).


We were running the fuel ratio at 32:1, which I was told by Bernie Richards(my Uncle)is quite old school. “Hey Bernie, Im an old school guy”. So we changed the ratio to 40:1, then I dropped the needle jet clip down to 4th, from 3rd(make it richer)position just to be safe. We also changed from Silkolene(which is supposed to be quite smokey)to Castrols’ latest 2T performance oil and super unleaded petrol.

A bit too much headshake at speeds, so tightened up headset to compensate.


So The Verdict?


Wow, what a difference!

The usable powerband increased to between 45%-90% without exhaust spacers

Then when we dropped those in, it incresed the low end to about 40%

In some places I was able to use a higher gear and use torque of bike and also roll on the throttle on some corners to keep up momentum before flicking clutch for launch time.

All in all, great gains for about £52(reeds & gasket set)and labour time.

Until the next Bike Tech, happy DIYing





18 Replies to “CR 125 2003 mods”

  1. Good stuff. I’m doin some similar mods on My 01 CR125. Suffers from some of the same issues if not more. I found some info from Eric Gorr about putting the 02 exhaust joint pipe on it. Also referred to an exhaust manifold, the shape is supposed to be the best out of the 3 years, 01, 02 and 03. Interested in those spacers you made, wonder if the manifold does the same thing??

  2. Hi David, I think what Gorr is referring to is the actual shape of it, meaning, and I thought I read this somewhere else, that the shape is wider. If I remember from looking at mine, the manifold would actually need widening to fit the contour of the barrel better. It is extremely hard metal though! Being that the bike is 10 years old, it’s now quite hard to compare parts. The one thing that I have found being a 5 speed gearbox is the ration between gears. It feels that either one gear is too high for the corner or the next down is too low. Gorr recommends putting in a 6 speed from a ’97. For now I’ve got a 12T front to try, as Gorr recommends and see how that is. Obviously, properly tuning the engine would help this, but I was trying to document mods that anyone could do. Let me know how you get on. Paul

  3. <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><html><head><meta content="text/html; charset=utf-8" http-equiv="Content-Type"></head><body>Good info Man. This Is a fantastic Blog, I am currently doing many of these mods. What are and where do these spacers go that you were adding, not real clear on that part. I read about the 12T sprocket as well and was going to try it, how has your experience been with it?<br/><br/>Turns out who ever had this thing apart last had the RC linkage mis aligned, that was a fun fix! <br/><br/>Hey if you are ever looking to give or get good Tech advice. There is a really good community on There are some really great people in the forums, really been around. <br/><br/>Thanks again! I’ll let you know how some of my mods work out! <br/><br/>Oh and check out the picture I attached, it was my alternative to drilling out the airbox. Just thought I’d share.<br/><br/>Dave<div>Sent via BlackBerry from T-Mobile</div></body></html>

  4. Hi David, thanks for your comments.Basically the spacers go between the cylinder and the expansion pipe, extending the expansion pipe. Think of them as really big washers, but obviously they don’t restrict any air flow. There is a company in USA that does them just made our own as stated before, so everyone can have a goThe metal we had handy was quite this, so we made 3 for a total of about 5mm in thickness. They seemed to add about another 5% oft torque to the low endThere was no pic attached?Have a great weekend. Paul

  5. <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><html><head><meta content="text/html; charset=utf-8" http-equiv="Content-Type"></head><body><div>Sent via BlackBerry from T-Mobile</div></body></html>

  6. <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><html><head><meta content="text/html; charset=utf-8" http-equiv="Content-Type"></head><body>Man I don’t know what’s up with that! Oh well, how did you like the 12 tooth front sprocket?<div>Sent via BlackBerry from T-Mobile</div></body></html>

  7. Hey man, got the pic now at my email.As of the 12T, I havent been able to test it yet at the weather has been so bad here all the local tracks have been closed!Hoping to get out in next couple of weeksI’ll keep you posted, if not check back inPaul

  8. Hey man, So I got that 2002 Exhaust joint today. Turns out that its 5mm longer than the stocker!! Not sure about the shape, looks a tidge bigger. Just thought I’d let you know. Getting my modded exhaust stop pin back Friday. I’ll let you know how it runs when I get it out!Dave

  9. What did u you to keep the 7mm bolt from coming out that you put in? Also do you kno the specs on the port and polish job? I really need to know I’ve done 1-4 just need 5-7

    1. Hi Bobby, the pin will stop against the inside of cover if you make it the same length as original
      For the porting and polishing, you just want to clean up any casting flaws in the ports and then polish up the exhaust port
      Hope this makes sense 🙂

  10. Hi having similar issues with my 2002 cr 125 going I try a few of these things on the weekend and see if I can improve the low end of the bike.

    All of your photos on this thread are gone is there any chance you could email me the photos so I can ref my work against them.

  11. Hi Martyn, the links are still there to the pics, so you can still click on them
    Not sure why they are not coming up as thumbnails though
    I have no idea where the originals are, so I cant email them to you sorry

    1. Thanks for the reply Pics are working.

      I have checked my flap valve and I have no stopper installed what was the overal length of the bolt you used as I have nothing to compare to.

      Did the flap valve mod make much of a difference on its own?

  12. I put the 7mm stop and boyesen rad valve jd jet kit crazy how hard it pulls gearing is 13 51 also the bike lugs with my cr250 but I’m veteran 125 rider for trails might be me lol do the mods they work.

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