Time to change wheels and ...

[Jens Eskildsen]

A slight rebore shouldn't affect the powervalve

[Calum]

@jens-eskildsen Yes it will.

If the bore is stock (getting rarer these days and not worth assuming) then no, you're right. But once you start going over the first rebore size, modifying the valve is a must.

To save confusing, I said modify the valve just as a precaution.

[Yegor]

Yes, the cylinder was rebored at least once. Now I'm looking where do that again. Thx !!

@Calum > ?

[Calum]

@yegor Yeah that's the pup.

[Yegor]

@Calum **thanks to everyone ** . One question ( maybe a little bit stupid) XD . What material the cylinder is made of ? And the cylinder sleeve ? ( Yamaha DT 125R )

The cylinder sleeve Iron?
Cylinder Aluminium ?

Have you tuned Yamaha cylinder ? ( admision / exhaust transfers etc ?)

Cheers !

[Calum]

@yegor YEah that's correct. I imagine the bore will be a steel alloy.

I've had my barrel ported by a tuner.

[declan]

@calum how much did that cost bud and can it just bolt back on no jet changes etc

[Calum]

@declan No, porting is going to drastically change the characteristics of a two stroke engine.

I wouldn't advise it on a daily, or on stock ignition.

You don't get something for nothing. I'm not going into it here, but the principle is, peak almost always alters performance elsewhere, usually torque.

There are exceptions where engines are poorly designed and modifications improve performance with no effect. But the end of it is, these engines are pretty well designed. When it comes to porting them, you're making a sacrafice somewhere else in the rev range for more power.

[Yegor]

Hello ) !
I will go tomorrow to the workshop with the cylinder for bore. I know is privileged information and long testing time but do you know where I can find information about tuning cylinder transfers, I think is good idea do something there before it will be bored. Maybe in the same workshop they do that for extra €, will ask.

all this marked with color dots can be modified/enlarged : ?

at the top of ALL the transfers: (?)

so, thank you again for the help, and gas gas !

[Calum]

Depending on how good your English reading is, I HIGHLY recommend Graham Bell's Two Stroke Performance Tuning.

That will give you a lot of knowledge on tuning two strokes.

I would NOT recommend porting the barrel, or altering the port timings, without a programmable CDI.

[Yegor]

@calum said in Time to change wheels and ...:

Graham Bell's Two Stroke Performance Tuning

Thanks for the tip Calum . also I have dowloaded the Two-Stroke TUNER'S HANDBOOK By Gordon Jennings. Time to read.
( http://www.2strokeengine.net/gordonjennings/twostroketunershandbook.php )

so, for now I'm going to polish a little bit where it is needed , also the power valve , because it is very dirty.
Mitaka pistons are good ? where you can buy original Yamaha DT R parts online ( like water pump complete repair kit) ? <--- have find this official site : http://www.recambios-yamaha.com/es/dt125r/2002/bomba-de-agua.html

Thank you very much everyone, your answers are very useful.

[Calum]

@yegor I'd recommend PJME, they're based in England but are Two Stroke Specialist, I believe they ship world wide.

Polishing will get you so far, gasket matching is going to be far more effective.

If you're going for performance, I'd avoid mitaka. Nothing wrong with them, but they are cheap CAST pistons designed to offer a cheap alternative to OEM.

For performance, go forged. Grab yourself a forged Wiesco piston and a set of chromium rings.

[Yegor]

@calum said in Time to change wheels and ...:

Grab yourself a forged Wiesco piston and a set of chromium rings.

can't find Wiseco ( one ring ?) piston for my model. But in PJME I have find Wossner piston ( 2 rings) , they are good I think, also forged (?)

Do you know if in PJME there Wiseco piston for the DT R ?

waiting answer from this site . http://www.paramimoto.com/pistones/343-wiseco-piston-yamaha-2-tiempos.html#/piston-dt_125_75_76_684m05600

[Calum]

@yegor Ahh yeah sorry mate, vertex and Wossner are perfectly good forged components, I run Wossner myself, but will be switiching to wiseco for my new engine I'm building.

For reliability, I'd stick to twin rings rather than just one.

[Yegor]

Hello friends !

Finnaly I've recived the piston. While they still fix the cylinder, I am reading carburation manuals and cleaning the Mikuni. I'll buy carburetor jet kit from 180 to 250.

I have noticed that the Yamaha piston rings are different while the Wossner ones are the same:

The Small End Bearing is 17mm ( the one that was put ) but the original bearing for Yamaha piston is 19.5 mm, so is very important replace it ?

And I see that the Wossner piston does not have holes that lubricate the gudgeon pin:

( it might be a good idea to do them? although I do not think so ) )

I have a base gasket but the Genuine Base Gasket it is made of metal (aluminium?) , so It is highly recommended put the metal one ?

uf a lot of questions but thank you in advance . :man_biking_tone1:

[declan]

@yegor as far as I know stock bikes don’t have the metal base gasket I wouldn’t bother with the lubrication hole if it was needed they would of already done it

[Calum]

OEM gaskets are far superior, but Athena gaskets are perfectly good. You MUST use either genuine or Athena for the head gasket. The base gasket doesn't matter.

Be sure to measure the piston ring end gaps on those wossner rings. I have never had a set that were perfect, they've always needed fettling.

[terry.tz]

@yegor said in Time to change wheels and ...:

http://www.2strokeengine.net/gordonjennings/twostroketunershandbook.php

I have read this and I would say read something else after reading it to put information in a better order for understanding

[Yegor]

@declan ok thanks, I think the same. But in PJME you can find the Genuine Yamaha base gasket and is the same that I have , made of metal ( I think steel )
http://www.pjme.co.uk/acatalog/4FU-11351-00.jpg

@Calum thx , I will pay attention to ring end gaps. And have bought the Athena head gasket and the original yamaha water pump radial shaft seal.

@terry-tz Thanks , will try read it in English ( I'm from Spain) , and find some literature in spanish.

Now I have disassembled the carburetor and I have understood much better how it works and how they connect the holes. this video helped me a lot:
Chicho Lorenzo ( Lorenzo's father MotoGP)
https://www.youtube.com/watch?v=niNn0LZESfg

My questions are. Our Mikuni carb ( TM 28 ) main original jet is 180 ?
in this webpage you can see specs and the main jet is 180 .
-> https://www.japanbaiku.com/mikuni/mikuni-tm/mikuni-tm28

. Now I have seen that I have 240 all this time , without problems. With the Romeu exhaust will try with 250/260. BUT this jet :
https://www.japanbaiku.com/image/cache/data/chicles_tornillos/kys-00b-500x500.jpg <--- ralenti/starter jet ? the original is 15 . will try 17.5

where do you buy jets kit ? www.japanbaiku.com is a little bit expensive.

Thanks in advance !

[Calum]

@yegor Hi bud,

240 main jet is fine, in your climate it probably about right. In colder climates then you would want to up jet slightly.

Jets are expensive, I cannot stress enough how important it is to buy genuine jets from reputable sellers.