@dominik22 I'd start by refitting the airbox snorkel and the stock silencer if you have one. My French '93 4BL ran very well running the jet sizes you mentioned (i.e 240 main) with either a full stock exhaust system or a Tecno expansion/DEP silencer combo but it didn't like the stock expansion/aftermarket silencer combination.

Sometimes the tailpipe can affect performance as the stock expansion chamber and silencer are designed to work together; if you change one it can alter back pressure and mess with the pressure waves going backwards and forwards. These evacuate exhaust gases from the cylinder, assist in drawing fresh mixture up through the transfer ports and act to keep the fresh charge from escaping straight out the exhaust port. I never thought the tailpipe alone would affect this but once I tried a brand new DEP silencer on my otherwise stock French '93 4BL with 3NC CDI; it immediately refused to rev beyond 9k. I rode it around for about an hour to eliminate wind direction etc., came home and refitted the stock silencer and straight away it revved to 10k+ again. This is only personal experience, I don't have access to a dyno or anything but it made a big impression on me. A Tecno expansion chamber would probably work well with that Athena silencer.

Tecno expansion is identical to the Athena apart from the clearcoat and under half the price. Only downside is the clearcoat melts around the header pipe and is difficult to sand off making your bike look like a Crème Brûlée. If I bought another one now I'd paintstripper it and paint it heatproof black before fitting it. Very good quality pipe for the money, gives a more usable mid-range hit than the DEP which delivers peak HP revs about 600rpm later.

https://www.ebay.co.uk/itm/253780922884?gclid=Cj0KCQjwurS3BhCGARIsADdUH5215jUowscJtBepB7NQYPYfR4AeBV1Gu63S4cHFVVEN8aPRotYawA4aAoc7EALw_wcB&_ul=GB&mkevt=1&mkcid=1&mkrid=710-53481-19255-0&campid=5339059258&toolid=10044&customid=Cj0KCQjwurS3BhCGARIsADdUH5215jUowscJtBepB7NQYPYfR4AeBV1Gu63S4cHFVVEN8aPRotYawA4aAoc7EALw_wcB

Power on this run was 16.98 bhp.
On other runs it was 17.58. Without any mods.

I hoon around town on an unrestricted DTR and it uses less than 5 L/100 km.

Good that you've confirmed that it's not the powervalve, that's one thing we can rule out.

First things first, bin off that coil. Get yourself a genuine 3RM one.

https://www.fowlersparts.co.uk/parts/view/4RP8231026

I had a horrendous issue that took me months to diagnose, since it would run absolutely fine from cold-warm. But after a short while it just refused to rev. It took me literally forever to figure out, I swapped loads of bits out and considering my bike was a "fresh" rebuild I didn't think it would be the "Racing Coil" I bought. It hit me when my brother was having problems with his RD I noticed he had the same type of coil and the tuner said bin that immediately and all his problems went away!

So first things first, get yourself a genuine coil.

Later coils are a different part number:
https://www.fowlersparts.co.uk/parts/view/3KJ8231013

So it might be that your coil isn't right for your bike/year. So just source the correct year/model coil for your bike and make sure it's a genuine one (Denso I believe makes them).

Secondly, I've seen this before whereby the exhaust got full of water after a DTR was stood outside for a long period of time and this had very similar behaviour. Running rich and lacking the ability to rev/pull hard.

So whip that exhaust off and give it a good clean, it was very odd and not something I had considered.

Later model DTs were fitted 210 main jet, but again this all depends on your carb that you're running and whether or not you're fitting genuine Mikuni Jets. Ideally you want to be fitting the right jet for the year/ignition etc.

@dominik22 Well you're doing better than me, once I was rebuilding a TDR250, thought I'd finished and then rode to my friend's house without connecting one of the carb warmer hoses! As you can imagine the temp gauge skyrocketed, luckily it wasn't far and we were able to top it up so I could get home.

Nice one, thanks for that suggestion! I’ll give that a go.

Ps. Nice name!

They are fuel overflow hoses. Attach a hose like shown on the other side and feed it underneath the bike away from any hot areas.

They're designed to flow gas away from the bike and hot areas should the bike fall over and the tap not be shut off.

@eric74 Fair play for getting on and replacing the tacho drive O-ring. Looking at your pics I'd say the oil is definitely coming from where the cable joins the tacho drive so I'd order the oil seal for this (item 7):

https://www.fowlersparts.co.uk/parts/4083282/dt125r-3py3-1991-999-a/tachometer-gear

Looking at the 4th image you posted up, the plastic breather stub should be right behind the tacho drive about where the old O-ring you mentioned is so it looks like your breather is missing (items 14 and 15):

https://www.fowlersparts.co.uk/parts/4083277/dt125r-3py3-1991-999-a/crankcase

Not sure whether oil would be visibly leaking out of here if it simply wasn't fitted (you never see any oil in the clear tube IIRC) so it may be that someone had the bright idea of plugging the hole with silicone sealant or something similar. Have a poke around here but be careful, if that's what's been done it would be too easy to end up with it disappearing into the crankcase, but if you're patient with the needle nose pliers/tweezers etc. you can remove a wodge of sealant like that from the outside.

It's doable. I've taken parts from 3 Mikuni 28s.One from my DT,one of a TDR250,one that has oil feed tube don't know what bike and made my current carb.
The one with broken is off,the bent tube is overflow if memory serves.
Chokes are the same.

@dominik22 I take it your pump is not the plunger type?

https://www.youtube.com/watch?v=WwFUzgr2xnA

This is how I set my pump many many years ago. I did not align the lines intentionally just to run a fraction more oil due to the porting work I was running at the time.

My servo do t seem to be getting any power it's not doing a cycle all the wires look good connections has me stopped myn is a 4 wore plug witha separate yellow wire it has me stupped

@StanKn

Mine was like it when I first sussed out it was removable. It was caked in what looked like cement dust!
I’m glad you got it running sweet again 😜

First gear, back brake on. I use a half inch socket with a slide bar, apply pressure to take up the ‘slack’ then a sharp tap with a rubber mallet usually does it…

Thanks for all the info sounds absolutely perfect for me, I'll definitely buy!

@erioni123

Like Callum said, use a flywheel puller to push the crank in and separate the cases. DO NOT use a screwdriver or chisel, please!…

A rubber mallet is usually enough for me…

@SpookDog

Ok i will try everything ……… I will find it 😂🤣

@Hark_Ptooie

Kinda pisses you off when your recently ‘rebored’ piston used to rattle back and forwards in the barrel, doesn’t it?! I had the same experience buying a REBORED barrel & piston.
I’m very particular about who does my rebores now…