DT125R FORUM

Nice work!
I'm thinking Yamaha should have angled the carb just a little more to the side to clear the shock, and have a more unrestricted path for the airbox boot. Just a few more degrees to the side..
I'm soon getting to this problem with my DT230 conversion, and I ordered a CR125 intake (same size as DT230, CR125 VF reed is bolt on) to see how that aims the carb.
The intake that came with the engine has a too large connection for the carbs I intend to try, but inner diameter is only abt 31mm, so really suitable for a 30mm carb. I also ordered another 'stock' intake that hopefully has the correct diameter for my carbs. There are two versions, and I got the one with the larger diameter carbs with the engine.

Could the ignition timing be off?

Not sure if this will work: https://www.facebook.com/photo/?fbid=10165850989140701&set=g.1650331705410143

Hard to get tuning parts for the LC engines though! I have a modified Athena 170 on a DT LC engine, It was all I could think of. Welded on a reed inlet on the cylinder to get a cylinder reed. Had to send it for re-plating after that, because it was warped during welding. Runs about the same as the DTR which also has the Athena 170 for now (until I transplant the 230 engine). It's a custom build DT LC 'supermoto style' with a low slung exhaust and 18" wheels front and rear. Yamaha competition yellow with some DYI side fairings and graphics. I named it DTX. Put a lot of work (and money) on that one this winter!

It has the kick starter shaft too, so I can just fit that if I want, but I want the electric starter.
The bushing for the mount under the swing arm is abt 1mm too long, so I had to grind off abt 0,5mm on each side of that one.
The intake has an enormous mounting diameter for the carb, so I ordered both a Cr125 and (hopefully) a smaller version of the DT230 intake. CR125 VF4 drops straight in, but might obscure the boost port too much. I have no plan to run e mega carb. I think 30-34mm should be enough.
I have taken off the cylinder now, and will measure the engine and simulate it in EngMod2T later on.
Expect no pictures here, because I don't use the image hosting services that go away, ransom your pictures etc.. (if you know, you know). I did put up some pictures in the Swedish DT125 FB-group.

Some comments:
The Athena exhaust has only a 19mm stinger, I replaced that with 22mm (ID) stinger, and it improved a bit in the mid to high revs. Also helps keep temperatures down. The silences also has a 'restriction' in the perforated pipe, so it would be good to replace that too (even if I didn't)
I found raising the cylinder also helps a bit. I found 1,5mm to be a bit too much, 0,5-1mm seems to work. You also need to adjust the head when raising it. I cut my own 'bath tub' chamber with abt 11:1 geometric CR, which seems to be a good compromise to avoid deto with pump gas. At the same time I changed the shape of the piston crown to a 'normal' shape, not the Athena compromise so they could use the same head as the 125 kit.. even the power valve seems to be the same, radius is not matching the 65mm bore..

I definitely recommend Zeel, it will give you the possibility to tune ignition to get some more over-rev, tune ypvs to the pipe etc.
Seems to be the transfers that limit the high rev power on the cylinder. Raising the cylinder improves this, because the transfers are far from fully open at BDC. However raising the cylinder also raises the exhaust to extreme durations (approaching 200deg when you start raising the cylinder), and that is not good. As is, the exhaust blowdown STA seems to have margin for more power, but the transfers don't flow enough to support it. So if you are going for porting, I would recommend starting with widening and raising the transfer port windows. I don't think there is much to gain from matching in the crank case etc, it might make you feel better but rarely has any real results unless it's really bad.

I finally received the engine today! Started test-fitting it in a spare DTR frame, and there is a issue with the starter 'features' hitting the frame under the flywheel. I also started replacing the stator, as I'm going to run it with Zeel, so needs a charging coil as mentioned before.
This will take some time, plan is to measure up the engine completely and get it into EngMod2T simulation software first. So, I need to lift the top end, and probably I will do some cleaning of the ports and modify the head, as they seem to be very low compression from factory. I will need to sim the engine to get an idea of what way to go with the exhaust, modify an Athena exhaust, modify a gasgas 250 exhaust I have, or something else.. That way I can enter the exhaust dimensions and simulate them before cutting and welding etc.

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.