04 125 problems!

Hey guys, my 125 is giving me some trouble an I'm hoping that u guys can give me some help with it, this post might be a little long so bear with me, thanks! About a month ago I blew up the bike so I replaced the top end which wasn't a problem, after breaking it in I go for a ride and the bike fouled 3 plugs on a single ride, it was always a fouler but never that bad. we took it home to tweak the jetting, we moved the needle out one position and the bike was running a lot crisper so I though it was good to go. About 2 weeks later my friends and I load up the bikes and head out to Kentucky on a road trip to do some riding. When we get there I could start the bike cold without the choke but I thought nothing of it so kept on riding. After about 50 miles of riding the bike quits on me going up a long hill. It slowly bogged down over a period of about 10 seconds so I though the plug went. I jump off throw In a new plug but when I go to start it it has no compression. next I checked spark and it had none. What do u guys think could be wrong? My hypothesis is that cdi went leaving the electronic power valve open therefore lowering the compression and that would explain why it has no spark. But I'm no expert so I would like to hear what u guys have to say! Sorry about the long winded post this bike is making me pretty angry so i will appreciate any insight into what's wrong, thanks!

Sounds like your ignition is going out. Weak spark means that the bike would foul plugs easy (this happened on my 03 yz250). When you played with jetting I bet it ran better (more crisp) when you dropped the needle or leaned the jetting out. Leaning it out makes it run better because the weak ignition doesn't have as much fuel to burn. But leaning it out also makes it run hot, aka melting pistons. So it wouldn't start when it's cold because you had the jetting too lean, and with the jetting lean overtime it probably melted your piston, or ring. And the power valve sticking open would just cause a loss of bottom end power, it should still have the same compression power valve open or closed. So it sounds like your ignition is going out and you might need a new top end.

I read wrong about the cold start, and the long 10 second bog you talked about was most likely the top end giving up, losing compression.

^ Just my initial thoughts. It would be nice to see what other members think.

Thanks for the info! I'm pulling the bike apart now and it has 0 compression and no spark. I'm pulling off the cylinder to get a look at the piston now.

It sounds that the ignition was about to die by leaning it you will deal with the symptoms but not the root cause. 


If you don't have a spark there is something wrong with the ignition including CDI.

By leaning it you might have destroyed the new piston, maybe you where lucky.


Open the engine and check the entire ignition cause of no spark.

even without compression you should have a spark.


Maybe check the PV Servo motor as well for proper operation. 


Good luck

I pulled the piston and was all messed up. The front looks like it was rubbing against the cylinder and melted the ring in, and the piston skirt looks like it was chewed up by a bunch of angry beavers. How would I I about checking all of the electrical stuff, I don't know to much about the electrical side of things. Also I forgot to mention that it was smoking a lot, and suckin up some tranny fluid so the crank seals are toast, would the left side crank seal being bad let to much air in a then cook the piston?

If the right went bad then you'd suck oil and foul plugs like crazy and if the left went bad it would suck air but I believe your jetting changes caused the issues along with weak spark like the other guys have said. When I got my 125 the right crank seal was bad and they guy before me leaned all the jets to try and counteract the tranny oil coming in so I did crank bearings and seals and put it all back together and I was in my third heat cycle and the engine cooked itself. ( I was too young to understand it was way way way too lean) so basically when a crank seal goes bad it's caused by the vibration of a bad crank bearing. My suggestion is, do crank bearings, seals, figure out the electrical issue and change your carb settings back to how it was when it had strong spark

Also a top end!! Wrist pin bearing too. When they get hot and stressed they get weak

Edited by Derfboy

Thanks for the help guys! Well it sounds like I am going to be redoing the whole motor, would it be a good idea to change the engine from the 04 to the 03 now? I remember reading in a different thread that the cases on the 2 motors were the same so changing to the earlier engine only required changing a few parts. Has anybody done this and know what parts need to be changed in order to turn the 04 motor into an 03?

If you want to change the you need a new top end.

Cylinder, Piston, Head and Power valve.

As well you need the governor form a 03. Might be possible for about 300$ if you are lucky. 


I bought a 06 cylinder with head and PV for less than 250$.


The 04 has everything to go back to 03 except those items.

If you want to go 05 its more complicated.


You need a lot more parts.


had a look at ebay this is a offer you could consider.




It seems with Pover valve, but just in case:




And the governor:




I just did a quick search I don't know those guys and these are not my offerings.

You can change it for less than 300$


the 04 Case has everything whats needed to change the engine to 03, but not 05 or later.


Good luck

Edited by pepe001

Thanks for the information, I think I'm going to change it to the 03 motor because I've heard that the 04 is one of the worst 125 motors out there and it will be hard to find parts for in the future, thanks again!

Just investigated a little bit there might be another change but not sure yet.

The Case is different where in the crankshaft area.


I have added a picture from a 03 and the grey hand painted area is how the 04 case looks like.

I guess this is one of the major reasons why the 04 is a bit weaker.


This can be fixed once the case is split, when I rebuild my 04 I left everything stock to see how it goes.

The 04 is the only newer CR125 with a crankcase like this.

When I rebuild the bottom end again I might change it I rode so many 125's and none of them had it like this. 



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