DT125R FORUM

Clutch can make noises too, for example if the rubbers between the gear and clutch cage have gone bad. Check the play if you remove the cover.

..and you need the battery too, not just the regulator. Forgot to write that

You need 12V DC for the servo. You could probably use a simple regulator/rectifier from a scooter that has a pin for battery charging. I use this type on a DT LC converted to 12V.

Something like this:
https://www.amazon.se/Universal-Motorcycle-Motorbike-motorcycle-rectification/dp/B09LMJP68D?source=ps-sl-shoppingads-lpcontext&ref_=fplfs&psc=1&smid=A2A8L8R3J2QTTW

You can take a piece of soldering tin and bend it and stick it through the plug hole to get a rough measurement of the squish. If the solder is not pinched you can 'double twist' it to get a reading. This should give you an idea if it's the cause. A good value should be 1mm or slightly less IMO. You can find different thickness base gaskets in some places, like here: https://www.motorsportgoetz.com/Cylinder-Base-Gasket-03mm-Yamaha-DT-TDR-TZR-125

Just measure from the inside of the fork leg to the rim on each side, +-2mm should be ok I think. If you release the nuts on the axle clamp on the right side (if I remember correctly) and pump the front suspension up and down, and then re-tighten, it should not bind. If the seals wear out too quickly, I would check the surface of the fork tubes, so they are not damaged (pitted from rust, flaking chrome etc.)

Check that the front wheel is centered in the fork, that should be enough. There should be no binding in the fork since one side is just a clamp.

My recommendation would be to keep the carb you have, if somebody has put a lot of time putting together a good setup, and everything is working well. Maybe you could tidy up the carb to make it look nicer.

It doesn't matter for resistance measurement, will show the same value both ways.

Then I guess it's ignition related, might be hard to find the problem..

You should get a set of main jets and go up in size significantly until it runs too rich, then start going down until you find the sweet spot. You might need to raise the needle too if it bogs when you twist the throttle. You will probably notice problems when running abt half throttle if the needle is wrong. Keep an eye on the plug when you are jetting, if you have some experience in jetting, the feel and sound of the engine will tell you when you are close. I sometimes use Chinese jet kits and think they work ok, especially for the price, but you can't compare the numbering on them with anything else, you have to stay with that kit when you are jetting.

So I ordered a chinese loncin 230 engine the other day:
https://www.alibaba.com/suppliersubdomainalibabacom/product-detail/subject-1601293337863.html?from=share&ckvia=share_5a6f391286154bb29332384b484f81aa&needReward=true

Price was abt 750EUR shipped to Sweden, specified to be everything paid, delivered to the door, but I doubt it.. Maybe VAT included, but not the customs fees is my guess. Still not too bad comparing to the price/performance ratio with a Athena 170-kit.
Shipping is by boat(classified as dangerous goods), so it will take abt 2 months to get here..

I have done some reading, and it seems people are using them in DT230's and also converting 125's. Might be an issue with the starter hitting the frame on my -88 DTR.
I also read that the stator needs to be replaced to work with a AC CDI like my Zeel, and the cure would be a Yamaha Majesty stator, so I ordered this:
https://www.aliexpress.com/item/1005006513896063.html?spm=a2g0o.order_list.order_list_main.10.34521802cdX2E8

The comments I found about the engines seem pretty positive, so I have some hopes.. I have not found specs on the gear ratios, just hoping it will be a wide ratio box. Other concern is vibrations even if I have not read any complaints.

Carb will be a 30 or 34 PWK copy to start with (I have them in my box of carbs), depends on the size of the rubber boot to the reeds.
I might even try it with the 28 I have on the bike now, just to be able to run the stock airbox boot.
Exhaust is unclear, I bought a used Gasgas 250 exhaust that has some damage, but I might even try it with the Athena exhaust I have on the bike now. Other recommendations I have seen are WR200 and KTM 200EXC that can be made to fit.

I will update with progress when I have installed the engine.

I use the Athena pipe with the 170 cylinder, it works quite well, and most of the power is in the midrange, even if I can get it to rev higher with modified ignition curve, the torque is in the midrange. I have seen Mick Abbey writing somewhere that the stinger ID needs to be at least 22mm (or was it 23) with the 170 kit, so that is a bit tight at least with the Athena pipe. The black paint seems to hold up quite well on the Athena-branded version.

I recommend Zeeltronic too.

@Calum
Is that undeslung exhaust a custom job? I'm building a DT LC 'X' (supermoto-inspired) and want to run a underslung exhaust on it. I'm thinking a underslung DTR exhaust could fit pretty well, some minor mods will probably be needed though.
I have a modified DTR Athena 170 cylinder with cylinder reed, and a Zeeltronic for that engine.

Could be a lot of things.. but everything is probably not as it should be if it's not starting..
What I can think of:
-Leaky crank seals
-Damaged/leaking reed/intake
-Carb issue
-Ignition angle off (flywheel key?)

Is the spark plug wet? Is there a good suction pulse from the carb when kicking it? With some experience you can feel it if you cover the carb with your hand on full throttle while kicking/cranking the engine. It should not blow, only suck, and the pulse should be 'strong enough'. Old fuel can make it hard to start too.

RD and DT LC are the same block, but gear ratios differ. RD has a close ratio gearbox and DT has a wide ratio. Close ratio is not ideal if you want to go offroad IMO. Primary drive with clutch and 'middle shaft' length differs in the the LC engines I compared. Definitely get a LC YPVS engine if you go that route, -more usable power than without power valve. I have a DT LC YPVS close to stock, I reshaped the head and installed a Zeeltronic and put in 0,3mm carbon reed petals, the rest is stock. Goes well and has really good midrange torque for a 125.

I think the DTR and TZR engines are the same block too, but there is probably the same gear ratio difference, and possibly more
I have not tried dropping in a DTR engine in a LC frame (have only one DTR engine and many LC's), but have thought about it. Let us know if you try it!

@HOTSHOT III
I found that the problem is delivery time when getting them directly, last time I asked it was months.. They are very good to deal with, and they were also willing to do a special version where the map-switch would change both ignition and power valve maps. I don't get why they have two PV maps, but only one is in use. I think both maps should be toggled when you 'flip the switch'. That would give two totally different engine characters with the map switch.

@eriondt125r
You can actually buy any PCDI-10VT and fit it on a DT, but it just takes more effort :) The CDI is the same, but connectors differ.
A 3-wire servo can be gutted and converted to 5-wire. There are also 5-wire chinese 'replica' servo's sold on aliexpress, but they are also a bit expensive (just under 100EUR form memory). I use gutted RC-servo's on two bikes with Zeel, and that works too. Basically it just needs two wires for the motor and three for the pot.
I use the RD350 Zeel on one bike (and a friend uses one too), and I ordered the same this time because it's cheaper :) However the RD350-version has no connectors, so you have to use some generic connectors and replace the originals on the bike.

Found this when looking for a Zeel:
https://www.motoracingshop.com/en/unit-zeeltronic-pcdi-10vt-dt125re-x-de06-con-programmable-ignition-power-valve-controller-for-dt125x-re-de06-with-plug-and-play-connector.html

Check ebay etc for used Zeeltronic maybe, but new that is about the price you have to pay.
Adding base gaskets will increase your port timings, maybe not a bad thing though, but an extra base gasket is a significant change on transfer duration.
You can weld and machine the 3BN head or get another one, but you will probably have to machine the other head too, to get a decent squish.
I have the Athena pipe, and it's better than the original (especially if the original is restricted), but also louder.