DT125R FORUM

Well, after some fiddling today I have a theory.
I installed a under-plug temp meter today. I even checked it against a known good temperature meter in hot water for good measure.
I went for a drive, and the under plug temp was in the 60 degC-range all the time during driving. The meter on the bike was then in the position between straight up and red. When I stopped and let it idle, the meter on the bike quickly climbed to the end of the green scale, but the under plug temp was just over 70 deg C. So the conclusion is that when the meter was just up in the red area, it was not actually overheating, but it must have been a small compression leak pushing the fluid out. No issues whatsoever during my drive today, even if a was pushing it a bit with some full throttle accelerations through the gears, and short high speed bursts. Easy 120kph with some pretty brutal head wind, I was feeling uncomfortable riding it at that point.
Don't want to push it too hard with the current carb setup, and I still have the 95oct gas in the tank too. Anyway, plug had a nice brown color afterwards.
Weather was a couple of degrees cooler today, but I still think the issue is gone. Under-plug temps are usually higher than the water temp, especially when pushing, so I'm really happy with it such low temps.

Some overheating issues this weekend. I have checked the thermostat, water pump and head gasket. I also blew out the cooling system with compressed air, but no debris found. The head was a bit warped (like bulging towards the middle of the sealing surface). However the head is not stock, it has been welded and a new combustion chamber shape was machined, so can't blame the manufacturer. I did not see any signs of head gasket leaks, so I doubt that was it. I resurfaced the head and put it back together. I'm running a bit tight squish at 1mm, so i might have to try lifting the cylinder with a thicker base gasket to increase squish a bit and decrease compression slightly. Should be 11:1 as is which has been ok on other bikes. I'm running double radiators, so cooling should be similar to the DT230. The first two tanks I did not experience any temp issues, so it's a bit strange. Outside temp was about the same as before. Only thing changed that I know of was (continuous) changes with the carb and filling it with 95 octane, since my 98oct fuel can was empty and there is no 98 where I live. I also made a ignition map with 2deg less advance to run the 95oct.

Like I said, videos did not work for me..

Congrats, looks nice! Cylinder has a iron sleeve, and can be rebored. Autolube might need adjusting with time, and replacing seals if it leaks. Try finding the repair manual online (Yamaha or Haynes), can be found on Facebook groups etc. Videos not available.

The 230 is up and running!
I have run about two tanks through it, but the chinese pwk carb I'm using is bad (had it in a box of junk carbs), so it drinks more than it should. Its stutters and putters on low throttle openings, and nothing I've done helps the issue completely (jets, needle and even filing the cutaway on the throttle slide). Runs well on more throttle though, and pulls pretty well. I have another clone pwk on the way, and hope that will work better (had good reviews). Internal gearing is higher, so 5th on the 230 is about the same as 6th on the 125. Went from 17 to 16 on the front sprocket, and now I'm doing just abt 5k at 80kph. It goes better than my sligtly modified 170-engine. I still have stuff to do with tuning the carb when I get it, tuning the zeeltronic, fixing the starter cicuit etc. I have not figured out how to integrate a starter button close to the throttle handle yet. Not a lot of space to add a switch next to the stop switch (that I'm using as a map switch now).
Engine seems fine, no problems with vibrations, shifts are good, and clutch is holding up. Some small drag from the clutch at standstill that can make it a bit hard to shift when standing still, even if the engagement point is fairly early on the travel of the lever. Using a DT LC clutch cable, because the DTR cable did not fit the engine, DT LC cable was a perfect fit.
I guess that's it for this update.

Would be quite a project to get one from Greece, picking it up, registration etc, probably a renovation too. It will be an expensive bike, but on the other hand, it can probably be sold for a higher price than a converted 125! If they get's too expensive, I'm reluctant to ride them because I might get a scratch etc. I thought they sold some in France too, but maybe not..

@Hark_Ptooie said:

As a swede who recently have ogled DT200's and Lanzas for sale in Greece, pondering if a trip down with a small lorry is too big a venture, I find this HIGHLY interesting. Please keep us updated!

Join the 'Yamaha DT125 Sverige' FB group, I try to update my post with some pictures from time to time. Plan is to get it running before the end of my vacation, in two weeks.

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.