DT125R FORUM

@Vuteng The previous owner may have just forgotten to reconnect it for some reason, if that fixes it then great. Connector is underneath the fuel tank as you say.

@Vuteng Yes that's right, just remove the two "R" clips under the tacho. All the bulb holders are rubber and just pull out of the tacho.

Great to hear you repaired the air filter holder!

@Vuteng No worries. You may be right about the pink seat, it's hard to keep track sometimes lol.

The oil light should illuminate with the neutral light, it might just be a bulb.

The air filter holder can be repaired with new foam. Get some 5mm air filter foam off eBay, clean the holder thoroughly and glue it on using Seal-All, a petrol and oil resistant glue. This is very important, if you run the bike without this the engine will suck dirt. It's also worth applying a dribble of foam filter oil right around it before sliding it back into the airbox with a fresh filter installed.

https://www.ebay.co.uk/itm/365607220379?_skw=air+filter+foam+sheet+5mm&itmmeta=01KXEHQNAYY94YVPA0ZCWEDJWT&hash=item551fe3749b:g:bw0AAOSwvgZoJ56X&itmprp=enc%3AAQALAAABAGfYFPkwiKCW4ZNSs2u11xBJIt2fFtFYVz7npvy9%2FdRi6S7mEgUJZasERX59eCBKfXenkOvpapeRTvdBtNGUKUfiPY4fEK4pf%2B9FZEvfYbBUON3xdL3tnhWs58zVRQNZ7pceGI5BgLatkYMECyxPveg5pYDIaV21EePCl7Cqfvwvk3ioK5viwwH2Yz8sIBO23aXzUAkNNyDVUNCXMZc3LzjGdqxuqVXOAKppI%2BXdoEEGNlsakeXqsPKIdfDmz%2BI2T5pOUti5Eev1OACioI8km0yJRYiIVkU2JROBBc1g6SRJx%2FMTcKF3eHF9UzWNeU7y5%2BvMJ4u3qBNGNE1dBrHcjSI%3D|tkp%3ABk9SR-DV3tHrZw

https://www.repairproducts.co.uk/product/seal-all-gas-oil-resistant-adhesive/

@Vuteng +1 for keeping this bike original, it looks great and the motor sounds good to me. I'd definitely enjoy riding it for the rest of the summer before taking on any serious work. In one of the vids it does sound like the rubber snorkel could be missing from the airbox intake, if you're somewhere in the EU the easiest way to get one is probably to just get a complete airbox from a French or German breaker which have plenty of DTRs. This model also shares the CDI unit and YPVS servo (both marked 3MB) with the KTM LC2 125 which sold well in Europe so if you want a spare CDI unit/servo that might be the best way to find these.

Looks like you have an OEM clutch cable, this and (especially) the throttle cable are best kept original as the throttle cable splitter has a mechanism that balances the carb and Autolube at all throttle openings. These are still available new and worth every penny.

This model I think had a dark blue seat when new (same colour as the frame/fork gaiters), probably available from bikerpartshop in Portugal:

https://www.bikerpartshop.com/en/166-dtr-50-125-200

Definitely retain the Autolube, and get a Haynes or Yamaha manual which explains how to bleed and adjust the oil pump. It's also well worth reconditioning it with a kit from Yambits. They used to sell a clear oil line kit but I can't find it, car windscreen washer hose works just as well. Here's a list of other bits to order at the same time, air filter elements are as good as OEM but half the price (get 3 or 4 so you can keep cleaning and re-oiling them and store them ready to change in a plastic bag, this is the easiest way to look after your engine), cable oiler (essential):

https://yambits.co.uk/dt125re-air-filter-p-9241.html

https://yambits.co.uk/dt125re-airbox-snorkel-genuine-p-230082.html

https://yambits.co.uk/dt125re-oil-pump-repair-kit-1989-1995-p-218588.html

https://yambits.co.uk/dt125re-oil-pump-oil-line-elbow-p-65302.html

https://yambits.co.uk/dt125re-cable-lubricating-tool-p-46571.html

Also here are a couple of things I like to do to any DTR; a full suspension linkage strip including making sure the troublesome relay arm bolt never seizes again, and making your own YPVS cables from MTB gear cables:

https://dt125r.co.uk/post/28245

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

@HannesDTX Check all the things @calum said, plus are you sure you've done a FULL carb clean? A lot of people just put them in an ultrasonic cleaner for half an hour and expect to bolt them back on. Doing this doesn't sort the blocked emulsion tube which is a leading cause of DTRs refusing to rev out beyond 6000rpm, precisely the symptoms your bike has.

Do a full carb clean including the emulsion tube (the brass tube the main jet screws into). A lot of people (including bike shops) miss this and don't realise it comes out because you have to screw the main jet back in without the brass washer and tap it gently to remove it from the carb body. The space between this and the carb body gets blocked with moisture, dirt etc and blocks more and more holes in the tube (which is how the air atomises the fuel) until it just starts fourstroking and won't rev out one day as this controls the mid-throttle fuelling. Just chucking it in an ultrasonic cleaner usually isn’t enough; you have to actually remove everything from the carb body, physically make sure all the jets and air passages are clear, then reassemble. Make sure the air supply to the emulsion tube is clear as well, it's the small brass tube on the right of the carb body just in front of the carb-to-airbox rubber. You should be able to blow through it, but only just.

@DTGREENLANER Nice job, that looks ace!!

@Strike78 I bought one of those Athena 125cc cylinder kits and was quite impressed with it.

The plated bore should deliver better heat transfer into the water jacket and spare A, B and C-spec pistons are also available from Athena specifically for this kit (these literally differ 0.01mm in size to compensate for the minimal plated bore wear over time and so are worth obtaining, @calum I wonder if these are available for your 421 kit?). It eliminates the head gasket issues the DTR is prone to and you also get a brand new powervalve and all mounting hardware so it's ready to bolt on. I can't comment on the performance as I sold the bike shortly after building the engine (which also included a Zeeltronic installation) but the new owner seems very happy with it.

Roughly the same cost as repairing a stock cylinder as you say but you do need to factor in getting one or two spare Athena gasket sets and I'd also be ordering B and C pistons (I think it's supplied with an A piston but do check this) as the fact Athena supply these shows they've really thought about the implications of a plated bore. This is something you need if you want to take the performance of any two-stroke beyond a certain level; you can port the stock cylinder all you want (some of the guys in Portugal even make their own exhaust sub-ports in stock DTR cylinders) but if you can't physically remove the heat from the cylinder, it will just keep blowing up. This is why all modern MX bikes and bikes like the Suxuki RGV250 and Kawasaki KR1-S (both around 60bhp stock) have plated bores. Also the 2MA parallel twin TZR250 (50bhp) was OK with steel liners but the Japan-only 2XT version (an attempt to bring the TZR250 up to RGV/KR1-S levels) had plated bores.

@DTGREENLANER Get some 5mm air filter foam off eBay, clean the holder thoroughly and glue it on using Seal-All, a petrol and oil resistant glue. This is very important, if you run the bike without this the engine will suck dirt. It's also worth applying a dribble of foam filter oil right around it before sliding it back into the airbox with a fresh filter installed.

https://www.ebay.co.uk/itm/365607220379?_skw=air+filter+foam+sheet+5mm&itmmeta=01KXEHQNAYY94YVPA0ZCWEDJWT&hash=item551fe3749b:g:bw0AAOSwvgZoJ56X&itmprp=enc%3AAQALAAABAGfYFPkwiKCW4ZNSs2u11xBJIt2fFtFYVz7npvy9%2FdRi6S7mEgUJZasERX59eCBKfXenkOvpapeRTvdBtNGUKUfiPY4fEK4pf%2B9FZEvfYbBUON3xdL3tnhWs58zVRQNZ7pceGI5BgLatkYMECyxPveg5pYDIaV21EePCl7Cqfvwvk3ioK5viwwH2Yz8sIBO23aXzUAkNNyDVUNCXMZc3LzjGdqxuqVXOAKppI%2BXdoEEGNlsakeXqsPKIdfDmz%2BI2T5pOUti5Eev1OACioI8km0yJRYiIVkU2JROBBc1g6SRJx%2FMTcKF3eHF9UzWNeU7y5%2BvMJ4u3qBNGNE1dBrHcjSI%3D|tkp%3ABk9SR-DV3tHrZw

https://www.repairproducts.co.uk/product/seal-all-gas-oil-resistant-adhesive/

@Hybrid_Theory01 Is it a genuine Nissin BRAKE FLUID level window you're looking at purchasing? If not I wouldn't bother because I'm not sure a generic oil level window off eBay etc. would be safe to use for any length of time in this application. DOT brake fluid is a solvent, not an oil and due to this it can seep through certain types of plastic and rubber at a molecular level.

Looking at your photo, this looks to be a completely blank master cylinder housing, i.e supplied without any of the parts these usually include. So you'll also have to accurately drill and tap the M6 and M3 holes for the handlebar clamp and reservoir cap (never easy with basic hand tools). You might also need to do this for the M10 banjo bolt fitting and it's absolutely crucial for this to be done perfectly to ensure safe operation of the brake. Plus the fact, on the DTR master cylinder there is also a very neat plastic baffle in the bottom of the reservoir which traps dirt before it can reach the piston and seal. It looks like this is missing as well.

Your best bet is to buy a good used DTR master cylinder from a breaker and rebuild it with a genuine Yamaha master cylinder repair kit (not cheap but worth every penny). Have a full tin of brake cleaner handy to blast the whole thing immaculately clean inside and out, including the aforementioned plastic baffle (Holts 600ml is best as the spray nozzle accepts the same size straw as GT85 etc. for hard-to-reach places), thoroughly scrub the underside of the reservoir cap with one of those miniature wire brushes to ensure it vents properly, and buy a new rubber membrane that fits under the cap whilst you're ordering the repair kit (the idea here is the vent holes in the cap allow the membrane to change shape and occupy more space inside the reservoir as the pads wear and the fluid level reduces). It's also worth having a really good circlip plier set to remove the old piston and fit the new one, and your blank one can help you here; you only get one chance to fit those piston assemblies so get the old one out and use that and your blank to have a few practice runs before tackling the new one. Drop the entire new piston/seal assembly into some new brake fluid and dribble more fluid down the piston bore in the master cylinder before you start, and just clean off the excess once you're 100% happy the piston is fitted correctly and working.

Also find a local fastener supplier that sells new M3 CSK screws for the reservoir cap and use plenty of copper grease when installing these as they're prone to seizing which makes them almost impossible to remove.

No-one ever takes good care of master cylinders but doing this in conjunction with a full caliper rebuild and decent replacement hoses can give you better-than-new brake action and total peace of mind.

@apg1991re @calum I've never owned a DTRe but the starter circuit does run through a relay, and that being activated is also dependent upon the clutch switch allowing power to reach the starter button (and obviously the button itself). I think that's where I'd be starting; being stuck on the end of a handlebar in our wonderful climate is a harsh environment for a switch. Give them both a good dose of WD40 so it fizzes and bubbles out of all the holes (hold a rag underneath them so none ends up on the front brake/tyre, it can also be helpful to leave a rag wrapped around it for a while afterwards). Do this with the ignition switch as well for good measure, there's usually a hole somewhere in the electrical part of the switch for this purpose.

@lucasg.03 Take off the stator cover and see if you can see a dribble of goop coming from the seal behind the flywheel (also possibly a good time to get a flywheel puller if you don't already have one, I think Yambits sell one). Once you take off the flywheel and stator you can replace the seal in situ as long as you're careful not to mark the sealing surface of the crankshaft.

@dan28 It connects to the black wire in the main loom which earths all the other components as well. Here's the 3NC wiring diagram from around 1989:

@lucasg.03 I wouldn't go for a Pipercross air filter element to be honest, the stock one is perfectly adequate if properly maintained. Interestingly I once bought a Twin Air DT125R element which was a bit too small to fit over the plastic cage properly, meaning the base wouldn't seat where it's supposed to seal against the airbox body.

I've also noticed some "tuners" on the DTR Facebook pages saying foam filter oil restricts power output. This is most definitely NOT the case; compare the surface area of the air filter element with the 28mm carburettor bore and there's no way a properly oiled filter could measurably reduce airflow.

You're better off getting 3-4 Yambits air filters which are just as good as stock but half the price; then establish a routine for cleaning and re-oiling them. Put the clean oiled ones in a plastic bag and you can quickly throw on a fresh one after every wash/off-road ride. Jet washing the bike and not changing the air filter/going for a short ride is a leading cause of ruined crankshafts on two-strokes, you only need like half a cup of water to creep in to knacker the mains/big end. And a clean air filter cuts down on a lot of other aggro; you see it all the time with RC car racing. Because a 1/8th buggy air filter is only the size of a 35mm film capsule it starts to affect throttle response after about 7 minutes on a dry dusty track. A lot of the guys at my local club used to try and play the Big I Am Tuning Wizard by endlessly twiddling the mixture screws but all they were doing was reducing the flow of fuel to match the reduced air flow (i.e less power). Whereas changing the air filter = getting your hands dirty; no cool points for the Bloke Down The Pub even though it means the engine just runs well and doesn't suck dirt right through the day.
Also the DTR airbox is poorly sealed where the carb-to-airbox boot meets the airbox (i.e downstream from the filter); it can be very helpful to seal this up with a product called Seal-All which is a petrol- and oil-resistant glue. You really need to remove the airbox completely to do this which means the shock needs to come out but this is easier than it sounds and you get to grease up the linkage bearings at the same time.

https://yambits.co.uk/dt125re-air-filter-p-9241.html

https://www.repairingproducts.co.uk/.../seal-all-gas-oil.../

@Strike78 Probably best you didn't fit the KTM headlight in the end.

Also, some LED bulbs fitted in lights not designed for them can disrupt the beam pattern and cause problems with the annual MOT test we have for roadworthyness in the UK, not sure where you're based but most countries in the world have something like this.

On the DTR one of the best modifications is to install a 100/80W halogen bulb which is a lot brighter than stock but doesn't cause an issue with the MOT test. I'm not too familiar with the DTX/RE headlight, @calum does it still require a P45T bulb like the DTR ort is it another fitting like H4 etc.? And do they have AC lighting like the DTR so they can still run a more powerful bulb without flattening the battery?

@Strike78 It certainly is! If you have an early DTR with the 3NC loom/CDI and 3-wire servo, by running a Zeeltronic and a 5-wire servo (more easily available) it's possible to install it without making any changes to your stock wiring loom. You can then keep the 3NC CDI and 3-wire servo and if you sell the bike or just want to return it to stock for any reason, it's just plug and play.

I'd just get it directly from Zeeltronic, Borut is very helpful and I ended up with quite a papertrail of emails after buying, he appeared genuinely interested in helping me gain a good understanding of ignition and powervalve maps (it's a lot more tunable if you run a 5-wire servo).

@Barron-1989 Yes white frame looking good

Get yourself on here for a 3-wire servo but be warned, it'll go crazy at the end of the auction, I'd be bidding in the last 30 seconds if it was me:

https://www.ebay.co.uk/itm/336567587789?_skw=Yamaha+DT125R+YPVS+Power+Valve+Servo+3+Wire+3MB-85820-00-00&itmmeta=01KQT8H9E0R37E9VYX7EPS2BN6&hash=item4e5cfdd3cd:g:bfgAAeSwr9dp-JC5

Other than that start looking around French and German breakers online as they're full of DTRs. The KTM LC2 125 trail bike (kind of an Austrian DTR) used the 3MB DT125RE engine and electrics and the 3-wire servo is the same so search for part numbers:

3MB-85820-00 (Yamaha)
51137095000 (KTM) https://www.trevorpopemotorcycles.co.uk/index.php?main_page=ktm_fiche_finder&model=112684&group=C96511304_080010&cat=3&bike_name=1998 125 LC2 100 (D)&section=1&groupname=CYLINDER HEAD EURO 125 96

I once purchased a number of BNIP LC2 servos which had both part numbers printed on the label and they were blatantly 3MB Yamaha 3-wire servos, sadly now all gone but if you're lucky you might find some somewhere.

Re PV pulley and cables, the pulley and housing is identical to all other Yamaha '80s/'90s two-strokes apart from the RD500LC so any pulley and housing from an RD350YPVS, TZR/TDR/R1-Z250 or TZR125/TZR125R/TDR125 should fit (they're all stamped 29L which is the YPVS350 model code).

I wouldn't recommend Yambits PV cables as the outers are very slightly the wrong length meaning you only get like 2-3mm thread engagement in the pulley housing with the PV adjusted correctly. Genuine cables are worth every penny or if you're into lubing cables after every wash like me, you can make your own here:

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

@Hoolies Yes the balance shaft only has one gear, assuming it's the same as the DTR balance shaft gears you just need to ensure the dot on the balance shaft gear is aligned with the dot on the crankshaft gear.

@Hoolies @beng1968 It does sound like the balance shaft gears aren't aligned, I didn't even know the pre-DTR engines had a balance shaft to be honest. This is where I went to find out, DT125LC MkIII is (I think) the closest I could think of, might be worth ordering items 24, 26 and 27 if you need to remove and refit the balance shaft gear:

https://www.fowlersparts.co.uk/parts/4047858/dt125lc-1hr-1987-999/crankshaft-piston

@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).