DT125R FORUM

@Elan-n Considering your symptoms the only thing I can really think of is incorrect float height; the only reason I'm suggesting this is, Haynes made a big mistake and got their VM26SS (round slide '88) float height (21mm) mixed up with the later TM28SS which should be 15.5-16.5mm. A lot of people read this and through no fault of their own, set their TM28SS float height to 21mm which of course means there's (a lot) less fuel in the float bowl at any given time. This could explain why the bike's pulling well when you wind on the power gently, but struggling when you apply full throttle straight away. Appreciate the plug colour might reflect a lean running condition but on the other hand it might not if you're experimenting with a lot of different engine speeds/throttle openings as a plug chop is only of any real use when done precisely at a specific engine speed according to a lot of two-stroke tuners.

Other than that, intermittent problems are often something simple; I'd go right through the electrics and clean up/WD40 every connector for starters. A corroded block connector somewhere can throw up all kinds of head-scratching like this. Do the handlebar switches, sidestand switch and the electrical part of the ignition switch (these usually have small hole for this purpose) as well, really blast the WD40 in so it fizzes and bubbles out of all the gaps and flushes out the corrosion as the exposure to all weathers on a bike is a harsh environment for a switch. Throw old towels over the wheels while you do it to protect the tyres and brakes.

I once lost the spark altogether on all 4 cylinders of a Suzuki GSF400 Bandit, it was a Japanese import with complicated V-Tec variable valve timing and no service information available in the UK so it caused me a measure of panic as you can imagine. The parts guy at the local Suzuki dealership simply said "a set of plugs (£10 each in 1998) will get it going". I thought you've got to be a genius to make 4 NGK plugs fail at exactly the same instant and so declined his offer. A bit later I was cleaning it and saw a relay under the seat (I still to this day don't know what it was for, poss. sidetand switch-related or something) which had some blue copper sulphate-looking corrosion oozing out of it, so I cut it open with a small hacksaw and found one of the tracks in the PCB had corroded through. Half an hour with the soldering iron, I bridged the split with a tiny wire out of one of those 8-pin landline phone connectors and it started up and ran perfectly.

You're going in the right direction with this, if it runs really well some of the time I don't think its jetting related.

@reudtr88 The seller is right, the spacers and caliper bracket are narrower than the DT125R versions.

The bench mock-up in the pics above comprised a 3ET DT200R swingarm, caliper hanger and spacers mated to a DT125R hub. As you can see the 428 sprockets lined up quite well (I also had to use a 3mm thick M20 washer between the caliper hanger and swingarm to get it as good as it could possibly be with the items I had to hand).

@kepirok I wouldn't use anything metallic like a piece of wire or drill bit to unblock the powerjet as it will be a very specific size. Did you try soaking it in alloy wheel cleaner? Check it out here from 6:52 onwards:

https://www.youtube.com/watch?v=-1ETYWSJNnM

@Calum Thanks bud, yes the TDR250 was great fun and released at a time when no-one in the UK really knew what Supermoto was. Getting the TZR250 wheels to fit was a popular mod at the time but quite difficult as there's no bearing available anywhere in the world with TZR front wheel OD and TDR spindle ID so I had to get a local machinist to make up some very thin sleeves which were carefully pressed into the TZR wheel to accomodate a different sized bearing. At a track day I met someone with another one who'd laced his OEM rear rim to the front hub to run a 120/17 size tyre, then fitted an even wider rim to the rear hub with a 150 size tyre on the back. It looked a lot more subtle than the 3-spokes and I think if I could do it all again that's what I would do.

@rob-tdr is correct, all the '90s two strokes are unbelievably expensive now, I've seen well-sorted TDR250s going for £8k+ on eBay.

@kepirok Hi bud,

I'd be looking at repairing your original carb or trying to source a replacement, if your cylinder is slightly ported as you say it shouldn't make much difference. Back in the day two-stroke tuners like Stan Stephens etc. used to wring a lot more power out of these engines, very rarely suggested a carb swap unless going fo a full race, rebuild-every-few-hours spec and in a lot of cases even recommended retaining the stock jetting. If your stock carb is really scabby try soaking it in alloy wheel cleaner for 24hrs.

The OEM DTR carb also has a slightly angled float bowl which means it's horizontal with the carb installed, a useful feature due to Mikuni designing the TM28SS specifically for quite a narrow range of Yamaha bikes. It can be a bit expensive to get one in good condition but the time and aggro saved (your bike will just work) is well worth it. The biggest problem with the TM28SS is the air screw seizing but this can be prevented by a smear of copper grease on the threads when installing it in the carb body (it's also worth doing this on all the other carb screws which retain the float bowl, carb top etc), and always spraying the entire engine/carb/radiator area with WD40 after washing the bike (get the 5 litre size with free trigger spray from a motor factors).

I think your Athena exhaust is the same as the Tecno expansion chamber which I've run on stock carb/jetting with good results. Is your ignition a Zeeltronic? If so they're very helpful and will probably tell you which ignition map works best with the stock carb if you email them.

@Rob-TDR Welcome to the Forum, that is a beautiful original TDR125R. They seem to be very rare and I prefer them to the TDR125 we received in the UK which looks like a TDM850.
Was the TDR250 sold in Italy? If so I think it would be easier to get one of those and keep your Lightburner original as it will be worth more as time passes.

The TDR250 is absolutely insane, I owned three of them in the 90s and they gave bikes four times their size a run for their money due to Supermoto agility and acceleration up to 70-80mph as fast as the 750s of the day. Everyone who rode one of mine came back unable to stop laughing, saying these things should be banned etc. Trust me, a TDR250 will always have enough power...

Benvenuto nel forum, quella è una bellissima TDR125R originale. Sembrano essere molto rare e le preferisco alla TDR125 che abbiamo ricevuto nel Regno Unito, che sembra una TDM850.
La TDR250 veniva venduta in Italia? In tal caso, penso che sarebbe più facile procurarsene una e mantenere la tua Lightburner originale, dato che il suo valore aumenterà col tempo.
La TDR250 è assolutamente pazzesca, ne ho possedute tre negli anni '90 e davano del filo da torcere a moto quattro volte più grandi grazie alla loro agilità da supermoto e all'accelerazione fino a 110-130 km/h, veloce quanto le 750 dell'epoca. Tutti quelli che hanno guidato una delle mie sono tornati a casa ridendo a crepapelle, dicendo che queste moto dovrebbero essere vietate, ecc. Credimi, una TDR250 avrà sempre potenza a sufficienza...

@Elan-n Don't worry about it, I've done a lot worse! Sometimes you can be so close to the thing you're trying to fix it's easy to miss something. The main thing is you're moving forward

@JOHNES No worries bud I'd still stick with an OEM coil if you can TBH, it's just one thing you can definitively rule out during the fault-finding process of elimination.

@JOHNES It sounds like the annoying spring steel retaining clip that holds the final delivery pipe onto the reed block; depending on how it was fitted, sometimes the end of these (which you need to get a tiny flat-bladed screwdriver underneath to remove) can be hidden and difficult to get to. It does give the appearance of the pipe fitting inside the reed block.

The pipe also sticks to the stub on the reed block over time so once you've got the clip off, try pushing it a little further on first to dislodge it. A clear oil delivery pipe kit from Yambits makes the bleed process a lot easier and gives at-a-glance peace of mind when riding, also comes with new clips:

https://yambits.co.uk/dt125r-oil-line-kit-clear-p-26778.html

Not sure why they don't list this for the DTRe, this was in the DT125R section but should fit.

@JOHNES Welcome to the Forum buddy, unfortunately those aftermarket ignition coils cause all manner of problems on the DTR/DTRe. New genuine from Fowlers (or your nearest Yamaha stockist if you're not in the UK) is worth every penny.

Are you sure it's the actual coil that is causing the problem, or could it be the HT lead? They break own on DTRs causing this exact problem. The actual coil is very good quality so it's well worth fitting a new lead. On early DTRs you can cut off the very top of the coil's outer case where the lead pushes in with a junior hacksaw, pull out the old lead and solder the new one to the coil output for a really good connection. Then re-waterproof using Shoe Goo and electrical tape. Looks messy but no-one can see it under the tank, and you should notice a performance increase as the leads deteriorate over time. Much better option than just buying a new pattern coil as these often have different specs to OEM which can cause other issues. Fit a new NGK cap while you're there, try to get the type that's moulded into a single piece of rubber.
I had a high-rpm misfire once, after trying everything else we replaced the HT lead in the manner described above which took around 1 hour on a summer's day so the engine was neither stone cold nor up to operating temperature when I took it for a test ride. It wheelied out of the driveway without me even trying, and I couldn't believe how much better the performance was at all engine speeds. Made a big impression on me as I was 17 at the time and I'd spent practically a weekend trying to fix the bike and getting nowhere.

@DTGREENLANER I'd say that's a dry bearing inside the servo somewhere, I think they can be taken apart and the gear train serviced. Never done one myself but it's pretty much a scaled-up RC car servo, just take your time and lay everything out in the order you removed it and you should be all good. Southeast Asia is full of DTRs, DT230 Lanzas etc. and some of those guys repair the servos quite often.

@rhodes5150 Pretty certain this is the one I used to run on the DTR, sticks out slightly more than original but the shape means it fits around even the more bulky DTRe engine cover:

https://www.ebay.co.uk/itm/363368465294?_skw=apico+YZ125+gear+lever&epid=8012759732&itmmeta=01KMDMK6F9YWWAGHQM3VVJ6WB4&hash=item549a72c38e:g:hPUAAOSw3gNggHtZ&itmprp=enc%3AAQALAAAA8GfYFPkwiKCW4ZNSs2u11xDUERQmESMU2CAlX1DcPGGjtQDWXKIOkMFlCI220%2FOghQ4QLH4yJSNvuMwJ6HtAxporHqFfRmGvzIVqQiIllh%2BKwfnrqpjXewQ7ymgSLsNdzsz32dKsKHRQckjfpclFoIXvLLsWkUu%2BdCjXGTj60n10tq0t8bgR%2BXtadCqU0hzv09UoU8ySJzIBIAWoLyKWQakbH0AINkprvtCDcGFBr5ahx7RWbK9OpZPk2z%2BX7%2FUNs4f3un%2FMQVgwueCuXdb2NDSpjumtBFuxFoGzhqF7QmVGJs6ODVThIgUhxBcNhhbS1g%3D%3D|tkp%3ABk9SR5rozLSjZw

@Elan-n Agree with @calum about the Zeeltronic, I enjoyed the process of installing mine and making it look as stock as possible.

Re jetting, I think I'd be contacting Romeu Henriques, tell him about your other mods (intake spacer dimensions etc.) and see what he says. Everyone speaks very highly of his exhaust systems so he will probably have enough knowledge to plug the numbers into some two-stroke tuning equations and help you out.

@Elan-n You can make your own using stainless MTB gear cables, use the inline cable oilers as well and you can quickly re-lube them after every wash:

https://dt125r.co.uk/post/28693

@Elan-n Have you done a full carb clean including the emulsion tube?

@madmackie No worries If you get the P45T bulb in the link it should fit straight into the light/wiring, P45T fitting is similar to the more common H4 but not identical. I don't know of any other Yamaha that uses P45T bulb fitting but every DTR I've ever owned has had it. Copied and pasted from Google AI Overview:

P45T and H4 are both dual-filament (high/low beam) headlight bulbs, but they differ primarily in their base diameter and application. P45T features a large 45mm round base often found on older classic vehicles, while H4 (specifically P43T) has a smaller 43mm base standard on modern cars.

Key Differences:
Base Diameter: P45T is 45mm; H4 (P43T) is 43mm.
Application: P45T is used for older, vintage European cars and motorcycles. H4 is for modern vehicles.
Interchangeability: They are not directly interchangeable, but adapters exist to fit H4 bulbs into P45T housings.
Performance: Both come in halogen, but H4 is more commonly found in higher-wattage, brighter variants.

Summary Table
Feature P45T (R2/H5) H4 (P43T)
Base Shape Large Round Smaller Round/Triangle
Base Diameter 45mm 43mm
Vehicle Age Classic/Vintage Modern
Availability Harder to find Ubiquitous

@Calum Thanks bud, I'd like to do more of them!

Now I'm no longer interested in riding it opens up the possibility of approaching bike builds dispassionately as engineering projects and I have a few ideas.

@madmackie I never bought a headlight cowl from BPS but the shrouds, side panels and mudguards I always found shiny and an extremely good fit.

I did get a few pattern headlight cowls from an eBay seller in Germany, they weren't as shiny as OEM as you say and I also had to modify them to make them fit perfectly as the part where you're supposed to drill the mounting holes doesn't extend far enough back. I found the neatest solution was to just cut off that part completely, make up some small tabs out of stainless steel and this enabled me to get the holes in exactly the right place.

With the headlight cowl, the biggest visual impact comes from getting it to fit around the headlight and front mudguard perfectly; this part of the bike looking "bodged together" leaps out at you. Another mistake people make is fitting the clocks above the upper fork clamp, then wondering why the lower headlight bracket mount (on the lower clamp) doesn't line up; this results in a huge gap between the cowl and the mudguard which screams "lack of attention to detail" from every angle.

With the German eBay headlight cowl modded as described above, I often rode my DTR to local Bike Nights where it generated a lot of "you don't see many of those any more" kind of interest, and no-one ever noticed the home-made mounts; people only really see what they want to see and with the graphics applied, clean bike etc. it's the overall effect that turns heads.

I do understand people wanting to make their DTR look perfect but equally there is a danger of the classic car "concours" mentality (they use shoe polish on the inner wheel arches!) detracting from one's enjoyment of the bike; Yamaha never really meant for the DTR to be like this and part of the enjoyment is knowing in one's own mind that the bike is as good mechanically as it can possibly be.

To this end I would focus on the following (others on the forum will tell you I always say this lol):

Run genuine clutch and throttle cables; these are extremely good quality and worth every penny, and the throttle cable on early bikes like yours (judging by the shape of your oil pump cover) has a mechanism which balances the carb and oil pump automatically.

Buy a cable oiler (£4.99) and keep them lubed after every wash for striking cobra responsiveness. A bit messy but again well worth it, Comma Spray Grease works well:

https://www.ebay.co.uk/itm/291576726736?chn=ps&_ul=GB&norover=1&mkevt=1&mkrid=710-134428-41853-0&mkcid=2&mkscid=101&itemid=291576726736&targetid=2446591572460&device=c&mktype=pla&googleloc=9219944&poi=&campaignid=23482766633&mkgroupid=187178521170&rlsatarget=pla-2446591572460&abcId=10587136&merchantid=101724959&gad_source=1&gad_campaignid=23482766633&gbraid=0AAAAAD_Lr1fLoLxzATg6uKloMef6pkxYF&gclid=Cj0KCQjw37nNBhDkARIsAEBGI8O3DDnU46NmtNww06wprGIu32YWAR6lcdBxjd2pbTPpXhP3hE3K5GEaAm-iEALw_wcB

Get a really good grease gun and keep lubing the rear suspension bearings.

Run a really good quality (D.I.D) non-O-ring chain and sprocket set; a sealed chain takes an extra 1/2 a horsepower to turn which makes even fully derestricted 125s feel sluggish. Unsealed chain requires a bit more timely maintenance but it's worth it for the riding experience.

Run a 100/80W halogen headlight bulb; the DTR has AC lighting which means it can handle this without flattening the battery unlike a lot of bigger bikes. This gives you rally car lighting and retains an MOT-friendly beam pattern:

https://www.ebay.co.uk/itm/161339938963?hash=item25909c1893:g:1~UAAOSwewJTn~QT&amdata=enc%3AAQAIAAAAwF8Gw72CnU%2FbU0Zeu1tjSRJFvfcr8SwbZT4fRnA6WLf6yF0iSX4F43bpTTUSgtfBlW1qXJSqMRtuS3Z1Luu%2FAD2VbsWrYvQrEc7oksN8VfDpaJHi6ZYW7zMtJnc%2F1be5np%2B6CZfap%2Frggb9cb01JyCQriOkPtIXXi8yWcKEn3mLr0%2Bhj6%2FoeZKr0o8Jl3vVA6Hka4%2F2xaD1awxGTbT2XEJFqjhxzyuYbuNe8sKHYOq%2Fho75%2FaGbmg%2FihQpqKGR%2B4kA%3D%3D|tkp%3ABk9SR6iwz8zLYg

@madmackie That bike looks ace, I've never seen that graphics kit before anywhere.

I think if it was mine I'd be getting another fuel tank in good condition, and a complete set of pattern plastics from bikerpartshop.com in the colour of your choice:

https://www.bikerpartshop.com/en/140-dtr-125-200

You can then store your valuable OEM plastics and either source another graphics kit the same as the one on the bike now or go your own way. One of the great things that makes the DTR such a usable classic is a good supply of brand new plastics, unlike all the other 90s 125cc trailies like the TS125R, KMX/KDX125 etc. where bodywaork is like hen's teeth. The only real difference between stock and pattern DTR panels is the leading edges of the rad shrouds are a slightly different shape but no-one but a card-carrying DTR trainspotter will ever notice this and it makes it easier to get out and enjoy the bike.

Glad you're enjoying the forum!

@Kristóf said in POS mode on switch:

So does this mean that with the main switch to ON and the lights switch to POS it should light up the auxiliary light even with the engine off?

No, lights switch to POS with the main switch to ON only illuminates the auxiliary light/tail light with the engine running because with the switches in these positions, the auxiliary and tail light receive an AC feed from the stator via the yellow/red wire.

@Kristóf said in POS mode on switch:

I'm asking this because my auxiliary light works when the main switch is to PARK but it doesn't work with the lights switch to POS

Yes, this is because with the main switch in the PARK position, the auxiliary and tail lights receive a DC feed directly from the battery via the red wire (which connects to blue/red at the main switch in PARK).

It took me a long time to fully understand this so glad I'm able to explain it properly!