DT125R FORUM

@Calum Oh no, it is not standard. The restrictions are gone and my crank seems to have longer stroke. 56 from standard 50,68 i think. This is where the problems started my brother said. Now I have also some holes in the airbox, a set of Boyesen reed-flaps and a pipe without a cat. The athena kit have 56mm bore, so that's pretty standard.

@SpookDog Well that is a recipe for disaster I'm afraid. It would lead to two holes in my piston if anything. If the engine is running lean it would sure not help to lean it even more? Smaller jets and a tighter fit for fuel in the atomiser sure would do so, would you disagree?

Thanks for all the input! The jetting required for derestricting (supposedly) is Mainjet from size 132 to 152 and raise the needle one step. my Mainjet is now 168, the needle raised one step, and I've also increased idle jet from 50 to 60 or so.

Carburettors are complicated devices it seems.It is clean as a whistle, and the thermostat is tested with most of all the other issues sorted out, I will raise the needle further one step to see what happens. Then it is necessary to get a new atomiser/ needle combo if I want more fuel in the main circuit.
The Aprilia rs125 use a smaller mainjet combined with a atomiser/needle that delivers more fuel for the high output versions.

Nice bike mate!

@Calum The jetting is very rich, I can not understand how this could be the problem as I've studied the settings for the aprilia rs 125 in different power outputs and compared to this one. Both bikes have more or less the same carb: dell orto phbh28.
But I have identified a new problem, the ypvs sevo marked yamaha is connected with wire 1 to the valve marked 1, and with wire 2 to the valve marked 2. As I did not check the operation before burning the piston and tearing down, this could have lead to confusion, and the valve closing at higher rpms.

This is the waterpump problem, no cover gasket resulted in contact with the metal plate and failure of the gear.

This is my project, my first motorcycle. The bike has not been running for the last 15 years or so because engine problems. I went to get it this summer at my uncles house. He had fitted a rd 125 piston with holes on the skirt. I burned a hole in that one before i washed the crankcase thorougly and put it together with an athena 125 kit. The other piston is from the time my brother had a engine seizure.

There is many possible reasons why this happened:
1.the stroke length being around 56mm giving me around 134ccm.(and different compression ratio)
2.water pump axle being jammed into crankcase causing a warped nylon gear and insufficient cooling.
3.Missing reed stoppers. I have boyesen dual stage reeds w/out stoppers.
4.A mutilated air filter box. There is extra holes in it.
5.air leakage somewhere
6.Lean jetting is probably not the case because I have really made sure to be on the safe side.
7. Aspen alkylate fuel. This does not have the required octane. This fuel did not cause the first seizure(as it was not available here at the time)

Now I decided to try again! This time with 98 octane premium fuel, rich oil mixture, and I got a cheap koso water temp gauge for safety.
I am going to fill the combustion chamber with oil at tdc, measure the volume and find the actual compression ratio. I have a original KTM expansion chamber without cat and restriction ring. The other restrictor was a plate behind the air filter.

Any hints or tips would be much appreciated.

@markus-w The KTM lc2 use a 520 chain on the water-cooled dtr 125 engine. Maybe you can fit a bigger front sprocket from this bike.

Greetings to all of you and thanks for loads of information on the dtr 125 engine. My project is my first bike, a 97 KTM lc2/ sting with the minarelli/Yamaha engine. Now I have been struggling a bit with this one, my piston got a hole in it and the plug electrode disappeared. You have some good knowledge here on the forum, really appreciate it and thanks to contributors. My problem seems to be a stroke length of 56mm, not 50,68. With my new 125 Athena kit I have measured squish being 1,1mm. The piston will almost open the exhaust port, half a mm being visible. Special thanks to Calum for posting the video of the bore of the 170 Athena kit. Really helpful video for me in this case. I will try post pictures and share what I learn from my mistakes with my KTM. Also, I own a 87 rd350 ypvs in bits and pieces, so please accept me as the true Yamaha enthusiast I am.