DT125R FORUM

Hi , Matthew . I had the exact same problem as you. My temporal solution was indeed to place a clip in the tube of the power jet, thus fully restricting it and fitting a 210 main jet. Even then, on full throttle I was running on the rich side, which made the engine to bog and not go above 8k rpm.

I managed to fix it some days ago. I will make a post with pics on the whole procedure sometime around the next month when I will have the time, but here’s some things I did to look into:

• Fully clean the carburetor body and jets using an ultrasonic cleaner.
• Replace all the O-rings with new ones.
• Replace the needle valve with the seat with brand new ones.
• Adjust the float height to the Hayens manual specifications.

After the rebuilt I went from running rich and having hesitation on fast openings of the throttle to actually running a tiny bit lean with crisp response .

*The lean is probably due to the 210 main jet I’m using instead of the factory 240 one. *

If you haven’t changed any of the jets, I think the above steps will help you.

Also be sure to make a plug chop using a new spark plug to verify that you are not running overly lean or rich before and after the rebuild .

@Calum Good to hear. Since a never ridden a factory new DT I can't tell if there is a difference with this oil. Anyway since I know it now, if I ever replace the clutch discs I will make sure to get the correct oil. Any other tips regarding the maintenance? I always had a fear regarding the big end bearing and the crank ones..., how do I know if they need change ?

@DTRLuke
Thanks for the suggestions Luke ! Yes , the brake fluid hasn’t being changed by me for sure. The braking pads I replaced them with brand new, and the discs came from my donor bike and are also almost new regarding the thickness.
Even though I checked them against a glass surface, I don’t have the equipment to check if they are bent.

Regarding the oil in the gearbox, when I got the bike it was over-flowing, and had a really dark color and smelled like burned car oil, so I drained it and flushed the gearbox with some clean oil a couple of times. But to my ignorance , and since I followed the manual to the letter, I just replaced it with a generic car engine oil to the manual specs ….. I found out you should not do that , after almost a year later , so at this point I don’t know what to do with it. Should I completely replace the clutch plates ? Can I clean them somehow ?

The autolube seems to be working on point, I never let the level drop too low, as not to introduce any bubbles in the system. It may actually be using a little more oil than necessary , as I see some deposits on the spark plug, but overall I need to refill it every 700Km if I remember correctly .

Hi ! I’m going to do a small chronical of my bike so I can hopefully give you some info for my upcoming questions , so bear with me

I bought a second hand Dt125r 1997 model, with 38K Kilometers on it at the time of the purchase.
Since then i replaced the cylinder ( cause the old one was scored due to piston breaking at the edge ) , the piston and the small bearing. During the rebuild I noticed some rust on one side of the crankshaft flywheels ( the bike was not used for more than a year before I bought it ) , I cleaned it as best as I could . I don’t remember if I checked for big bearing play and if I did , I did it by feel and not found anything. Also I replaced the Mikuni vm26 that the bike came with due to physical damage ( previous owner mentioned the original carb got stolen and the vm26 was the replacement ) , with a Mikuni TM28ss from a donor bike I had laying around.
Front and real brake discs got changed from the donor bike as well. The exhaust system is a DEP pipe , with an arrow silencer . I also serviced both the expansion pipe , de-carboned it and repainted it ( it was covered in rust ), and silencer.

After the “rebuilt” the bike was used for one year with almost no issues , it has travelled 2300 km, mostly in city riding, including a 270km trip at 110km/h cruising speed for almost 2h ( with breaks in between so I can feel my a** again )

Now to the point of this :

• At 40k + kilometers, what should I check as maintenance ?

• Is there some parts in the engine and the bike overall that I definitely need to replace or check ?

• What is considered a weak point for these bikes ? Meaning , what's the first thing to probably fail ?

Some issues I noticed over the year :

• The carb overflows if you leave the tank valve open and park the bike , and the fueling feels to the rich side. Also if you snap the throttle from idling it bogs for a second before it revs up. For the carb , I’m going to order the needle and seat and the bowl gasket . Also probably buy an ultrasound cleaning machine and clean the carb with it.

• There are some metallic ringing noises when riding , like if you leave some change in your car and they vibrate to the engine, but way more subtle.

• The overall engine noise seems to take a higher frequency aggressively when the engine gets into the exhaust tuning rpm ( around 6500- 7500rpm ). You can hear the change between 5000 and 6500 clearly ( almost like a small supercharger gets activated after 6000 )

• The front brake feels like it’s having two grip points as you press the lever, in low speeds the first “point” is enough to gently slow down but if you press a little more it grips suddenly harder. It’s like the breaking is not linear.

• Back brake works smooth, but it has a long travel in the foot pedal till it becomes effective.
  That’s all I can think of right now. Ask me freely for more details.

Another update ( kind of ). I was driving home at night , it was cold ( 15 C from 29 C in the midday ) and the bike had the most responsive throttle response I’ve seen :face_with_open_mouth: . It was like a rocket revving from idle till WOT with no problems at all. I guess it was the temperature ? One change i also made before the drive was i emptied the oil tank ( it was almost empty anyways ) and filled it with Motul 710 2T. I’ll make more observations in the following days…

UPDATE – First of all, yes a working battery gives the bike a lot more torque after 5-6k rpm, I think as well that the YPVS servo is not working correctly without a steady 12v source, or / and , the ignition is not working correctly ( thus the loss of torque ) .

Secondly, the 240 main jet was a disaster , it was like pulling the choke after ¾ of throttle , nevertheless I took a pic of the sparkplug after 15 secs at WOT :

Now I have fitted a 210 main jet. I run some tests and this is what the spark plug looks after 10 mins casual driving and then 15secs at WOT :

I noticed that now approaching 4/4 throttle I had some cuts that I think it was misfire ( it sounded like an interruption of the normal sound for a split second, it had a muffled note not metallic or pinging ). This may be due to the spark plug being fouled from the previous over-rich condition. Still, going from 3/4 to 4/4 seems like you are still holding ¾, there is no more ( or very little ) acceleration and the note of the engine gets a tiny bit more “ heavy”. It does not bog like it did with the 240.

@2stroketerry Ok , so how i know if the stroke of the cranksaft is the same in both engines. *** And on that note, if it was different what would happen ? With the exception of bogging at WOT the engine works OK ( i think.., keep in mind this is my first 2-stroke bike and my first bike overall ) .

Also DAMN that's a squeeky clean engine you have there !

I found a pic, the crankshaft cover on the 1997 bike reads ( 3BN03 )
----------------------------------------------------------------------------- ( Y-1 )

and from the donor bike it reads ( 3BN03 )
--------------------------------------------(Y-2)
What the second code means anyway ?

@2stroketerry If I remember correctly both cylinders had the same number and the same heads ( the 3BN type with the non-flat squishband ) . Now regarding the markings on the crankshaft cover I think they were a little different ( something like Y-1 , and Y-2 ? I’m not sure about this I need to call my brother to check as the donor engine is on a different town )

Btw, I was wandering why if the engine is the same they have different jets fitted on the same carburetor through the years ?
Or the engines are not so similar and I’ve made a huge mistake interchanging the cylinders ( although after 300k kilometers nothing exploded, even at 10k rpms :smiling_face_with_halo: )

@2stroketerry Oh , yes, I failed to mention that I run a mikuni TM28ss that was taken from a newer Dt. From the newer engine I took the cylinder ( with the YPVS and servo ), the carburetor and I installed them on my 1997 Dt.

Mine had scored the cylinder and somehow the previous owner managed to bend the air way of the carb, that be the reason i replaced them from the back-up engine i have.

@2stroketerry I have the same voltage reading without the battery, with battery fitted it drops a tiny bit to 15 V.

Since the problem only occurs at WOT , I’m leaning towards rich main jet , even though it’s a 240 as stated on manual .

I managed to charge the defective cell ( kind of, it holds 2.1v with a 2 amps load ), so I will run a test with the battery charged. If it still bogs at WOT, I will fit a 220 jet and re-test.

@Calum It will be my priority tomorrow. I’ll fit a new battery, test the response of the engine under all throttle positions and then if necessary I’ll lean the main jet . I will of course update the thread when I have more information :winking_face:

@Calum Hmmm, I measured the voltages at both the charging and the lights outputs. They are exactly on specs. I have no other idea on how to check if the ignition pulses are correct ..

If it’s not the battery , then I’m looking again at the rich problem regarding the main jet.

@Calum Yeah, I was thinking the same… I am attempting to do a single cell charge to try and revive the battery . I’ll look for a new one tomorrow anyways .

QUESTION : Will a dead battery affect the performance of the bike at WOT ? .

On my way driving home, I noticed my horn was not working ( when a real gentleman driving an Audi decided to cut in my lane , as I was accelerating, with no flash/warning :face_with_steam_from_nose: ) and after I arrived home I checked the battery to find a dead cell. The max voltage of the battery under 0.5 Amp load was a steady 10,3v. No load voltage of 12.6v.

In the same day , before I noticed the battery issue, I replaced the main jet to the manual recommended 240 ( from the 190 I was running as a test) and the bike starts to bog the moment you pass the ¾ of the throttle.

So, will the dead battery make my bike feel over-rich as I go WOT or the two things are not connected ?

@NINJA Thanks ! No worries ,I actually visited that thread when I was browsing the site

@Calum I took the cylinder to a well known shop for doing honing/ reboring , etc ( in Greece ) , he actually checked the cylinder for clearances as well as condition ( for almost 40 mins ) and then he gave me the correct piston. So I guess ( hope ) that honing is not the issue. I wouldn’t say I get excessive smoke. It smokes in the beginning and under hard acceleration , no more than what I see on youtube with this bike..

About the crank seals I have no idea, I check the oil level when warm and it seems the same. Unfortunately with the university, the only period of the year I can do the bottom end is the next summer. ( I’ll probably do it anyway then cause the old one had a little surface rust on the counterweights and I want to replace the bearing as well , i just hope it doesn't give up in this year :smiling_face_with_halo: )

@NINJA I lost you there.., i don't see something in this thread ( this is my first thread on this site , so i may miss something ).

First thanks for your replies ! Now it’s a DT 125 R 1997 , with mikuni flatside TM28SS. I bought it second hand , the piston was breaking apart ( small pieces had broken up and scored the cylinder but it was still running no problem ). Anyway I exchange the cylinder from a second Dt engine I have, the cylinder has 8000 km on it and just to be on the safe side I took it to a shop , they checked it out and gave me a new piston, rings , small bearing and Athena gaskets.

This is my try to tune the carb , after the top end rebuild ( I also followed the info from this side on how to do a top end rebuild and run-in ). The snorkel is in, but it’s not a tight fit with the air box.. ( I mean it has some play, it’s not like a solid piece of rubber ).

Regarding the oil pump, I checked it following the Haynes manual an I thinks it’s ok. ( I don’t have the specific tool for this so I measured some pieces of copper foil with the caliper and used that as a reference ).

The exhaust is stock with no obvious restriction. On more thing to consider regarding the oil is that I still run the one that the bike came with… Unknown age or brand…

The other setting of the carb are stock as the rest of the engine. It idles stable at 1.5k rpm and the revs drop according to the throttle position so I don’t think It’s leaking air ( at least not in a noticeable way ) . The rest of the engine and the bike has 38700 km on it .