DT125R FORUM

@Karmona @calum @madgyver Interesting thread, as I've never run an Athena 170cc kit I can only agree with all the above comments.

One thing I would like to add though is you might like to consider a Tecno expansion chamber, possibly as well as the RH and DEP pipes. These are relatively inexpensive and from what I can see are basically an Athena pipe (which your above link recommends with the Athena 170 kit) with a clearcoat finish instead of black or chrome plating. A few years ago I ran one with a DEP silencer on an otherwise stock 1993 3NC French import and it made the power come in about 1000rpm earlier than stock with a sharp cutoff at about 9000rpm. I did some measuring of that pipe and a DEP I also had at the time and according to the A. Graham Bell two-stroke tuning book, all other things being unchanged, the Tecno delivers peak HP revs about 600rpm earlier than the DEP (basically my riding experience on the 125) whereas the DEP allows more overrev. So if you want a 170cc midrange monster for wheelies etc. it might just be the ticket, and it would be interesting if you could establish the correct jetting etc for all three pipes once you've built the engine.

Only thing I don't like about the Tecno is the clearcoat which bubbles up around the header pipe and is very difficult to remove. Eventually I managed it using rags soaked in paint thinner just wrapped around the pipe and left for a while (do this outdoors!) before wire brushing but I had to do this a few times to remove it all. I wish they wouldn't do this as it actually looks quite "factory" with the bare metal/welding heat marks:

https://www.ebay.co.uk/itm/233679367459#:~:text=Item description from the seller,41%2C PH 49-50.&text=THIS IS A STANDARD PIPE,WILL NEED A COMPATIBLE SILENCER.&text=WE WILL COMBINE POSTAGE COSTS IF MORE THAN ONE ITEM IS BOUGHT.

@aragian Hi Bud and welcome to the Forum.

Sorry to hear your bike keeps killing CDI units, I owned a couple of 3NC DTRs and never had an issue but one of them, despite being an immaculate, low mileage example ran very badly when I got it and this was eventually traced to a pattern coil (among other things like incorrect jetting etc.). Those cheap pattern coils cause a lot of running problems on DTRs as they sometimes don't have the correct primary and secondary resistances, and in extreme cases this can damage the CDI so it's well worth taking a look at. Signs to look for (besides testing with a multimeter) include an overly long HT lead and being brightly coloured (OEM coils are black). If this is the case and you still have your original, best course of action is to replace the HT lead (explanation copied and pasted below):

Possibly could be the HT lead as 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; 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.

I don't think any other DTR CDI unit can be fitted to a 3NC without serious alterations to or replacing the loom unfortunately (3MB CDIs are similar and appear to give a stronger spark than 3NCs but still have one or two different colour wires so I wouldn't advise it). In fact a company called The Tuning Works who import Zeeltronic ignitions into the UK once told me the stopped selling Zeeltronic CDIs for the DTR because Yamaha changed the electrical platform so many times in its 20+ year production run (although Zeeltronic will sell you one direct if you email them pictures of your existing CDI/loom connectors). I just had a quick Google and was pretty shocked at the 3NC CDI prices on eBay etc. but if it helps you this breaker in Germany appears to have two 3NC CDIs and a loom which might be worth having (it's a 3RM loom which I think = 3NC sold in Germany as the loom appears to have the correct connections for a 3NC CDI):

https://www.ebay.co.uk/itm/146868783391

https://www.ebay.co.uk/itm/157439162265

https://www.ebay.co.uk/itm/146948681784

There are specialists in the UK that mend CDI units, so if you have a few toasted OEM ones it might be worth looking into this by asking around local bike dealers etc.

As I say I don't have any experience of repeated CDI failures but it sounds like something is overloading it, either a pattern coil with resistances too high or something at the stator end. Early DTRs have strange electrical system where the same wire (yellow/red) powers the lights directly with AC (this gives the advantage of being able to run a 100/80W headlight bulb which a lot of bigger bikes can't do, see below) and charges the battery with DC via the regulator/rectifier. The stator has three coils; charging coil (battery), lighting coil and source coil which powers the the CDI (in fact four if you count the pulse coil on the outside of the flywheel which sends the CDI the signal every crank revolution like the crank sensor on a car). Haynes manual has all the numbers enabling you to test all these as well as the ignition coil so it's well worth getting yourself a flywheel puller and a multimeter and going through this; it could be as simple as getting another 3NC stator and bolting it on (Yamaha used to sell the stator coils separately but they're hard to find now and IIRC you have to solder them up yourself).

Understand the attraction of getting a modern 450 but these aren't without their own problems; a lot of people in the MX/Enduro world are utterly fed up with the "four stroke revolution" as they can be complicated and expensive to fix when they blow up. If you persevere with this reach a definitive conclusion/remedy your DTR will be reliable and easy to keep that way for the forseeable future, a usable classic with advantages such as VERY powerful headlight, Autolube (literally mechanical engine management from the 1970s), limitless brand new plastics from Portugal and ease of maintenance due to grease nipples everywhere etc.

Here's a 3NC wiring diagram if you don't already have it, hope this helps:

100/80W Headlight Bulb Explanation:

The stock DTR headlight takes a P45T bulb (similar fitting to H4 but not the same) and has AC lighting (i.e the power for the lights comes straight from the magneto without involving the battery) so you can run a 100/80W halogen bulb without flattening the battery unlike a lot of bigger bikes which run DC. The DTR’s actual light lens/reflector is the same as all Yamaha superbikes of the 80s like the RD500LC and XJ900, but Yamaha cheaped out and fitted a 45/40W candle to the DTR which is why lots of people don’t find it bright enough. 100/80W bulb lights up the roads/trails like a rally car and retains an MOT-friendly beam pattern. Once a year or two ago I met up with a bunch of guys greenlaning on modern KTMs and decided to tag along with them as it was getting dark; they couldn’t believe how much brighter my headlight was. Get this one:

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

Also MOT stations have a headlight beam board and can set up the headlight perfectly for you in a few minutes but you can get it more or less where it should be by finding a brick wall and measuring the height the centre of the headlight glass comes up to with you sat on the bike. Then sit on the bike about 6 feet from the wall and get the headlight main beam to this level and pointing straight ahead using the adjusters.

@YzDt85 If you mean the DT200R 3ET alloy swingarm, as far as I know it's the same overall length and uses the same pivot/linkage bearings as the DT125R steel arm (and takes the same All Balls 28-1212 pivot bearing kit). Here are some pics of a bench mock-up I did a couple of years ago using DT125R crankcases and rear hub mated to 3ET alloy swingarm, rear wheel spacers and caliper hanger. 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).

IMO the engine boss at the rear of the crankcases dictates sprocket alignment probably more than the frame, and I reasoned if the sprockets aligned with the original hub the wheels are probably in alignment as well (although having no roadworthy DT125R at the time meant I had no way to check this).

Don't forget the all-important swingarm side clearance, it's a lot easier than Yamaha makes it look and if you get this right you can get the arm to fall under it's own weight with <1mm side-to-side play which massively improves suspension compliance and chain/sprocket wear:

Top DT200R tip; the plastic mud flap that protects the rear shock wears the (basically priceless) alloy swingarm away at the rear of the linkage bearing over time so it can be helpful to install something to prevent this. A lot of people with modern Huskies, KTMs etc. JB Weld a coin to the surface of the arm as they're prone to it as well.
I considered all that mud and chain lube floating about to be quite a harsh environment in which to try and glue two pieces of metal together so on my 250EXC I used the plastic sleeve which you get inside one of those boxes of sticky labels they sell in the Post Office cut open and cable tied on.

@Calum This is an amazing project, looking forward to the next instalment!

It's crazy how Yamaha making the 350 Banshee has breathed new life into the 350LC/YPVS community. This might be of interest, over on 999Lazer YouTube channel they're covering an auction of someone's entire collection which includes a brand new Banshee still in the crate: https://www.youtube.com/watch?v=E_mb4AUi4kg

Merry Christmas everyone!

@Calum @raceraidan Interesting thread.

I once owned a 1998 DT125RE French import which looked identical to a DT125R the same year (pic 1). It had a larger battery and separate toolikt compartment as well as the electric start wiring.

During this time I was fortunate enough to source a Yamaha service manual for this exact model, I still have this and here are some pictures of the wiring diagram. I believe KTM also used this engine/electrical platform for the LC2 125 which they sold a lot of in Germany (I bought 2 or 3 3MB CDI units marked LC2 125 from German breakers, and also some NIP 3MB YPVS servos which had both the Yamaha and KTM part numbers on the packaging (sadly now all sold). During my ownership I tried all these items on the bike and they all just plugged into the loom (same connectors) and worked. I'm pretty certain the (inverted) flywheel and stator were both stamped 3MB.

As you can see this model also has a clutch switch which IIRC, with the clutch pulled in, allows you to start the engine in gear even with the sidestand down, but cuts the ignition when you let out the clutch to try and pull away. If you've already looked at the sidestand switch I'd locate this and give it a dose of WD40 etc. as a starting point. Also the two relays (items 12 and 13) are worth a look although I'm not sure how you'd go about testing these.

It might be of some help to just get some more and plug them in and try them. Here's a link to the KTM parts fiche; search Google/eBay etc. for the KTM part numbers as well as Yamaha France and you might get lucky with a European breaker as they can be quite diligent with posting up used parts for sale including the original numbers:

https://www.trevorpopemotorcycles.co.uk/index.php?main_page=ktm_fiche_finder

https://ypec-sss.yamaha-motor.co.jp/ypec/ypec/b2c/html5/app/en_GB/parts-search/index.html?ver=bd30d8f2-88bf-45bf-81ba-b46167a61f3c

Here are some pics of the wiring diagram, not sure if the Gilera SC125 used the same electrical platform but hope this helps:

@ChuckSR86 I think the easiest way of doing this would be to just obtain a complete DTR or DTRe front end, rear wheel (and possibly swingarm/rear brake assembly) and shock. All these items are fairly common on eBay etc and are easier to swap out than you might think (I once had to replace a DTR swingarm when I'd arranged to go out riding the next day and got a spare one I had fitted in an evening, including the all-important swingarm side clearance).

Regarding the shock, I've heard some people say the DTX shock is shorter than the DTR/DTRe, I'm not sure about this but I once bought a few "DTR" shocks from a breaker which included one a few mm shorter than all the others but otherwise identical (maybe @calum or another DTX owner can confirm this?).

Running DTR forks mean you'll get to use fork gaiters which massively increase seal life off-road, and the whole operation will be a lot easier than getting into one-off bespoke front end conversions etc. which can be a bit of a nightmare when it comes to stuff like sorting a speedo drive and maintaining forks where the seller might have been a bit economical with the truth regarding what year of bike they came from.

Never owned or worked on a DTX so can anyone confirm if this is indeed straightforward?

@Calum Yes @SpookDog runs the Hyperpro fork springs IIRC and is happy with them.

I weigh around 150lbs and to make my '98 DTR a bit better off-road, my suspension setup was:

YSS adjustable rear shock and Lust Racing 25mm jack-up kit (the YSS shock is slightly shorter than OEM and the Lust Racing kit corrects this).

XT600 3AJ fork springs (20% stiffer than stock) and 15W oil (stock is 10W); this is a well-known DTR upgrade for greenlaning etc. and helps the bike jump better and resist bottoming. I adjusted the spring preload/rebound on the YSS shock to suit and the bike jumped better than the 2020 KTM 250EXC I also owned at the time.

I've also heard of people swapping on an XT600E rear shock spring, this apparently sorts the spring rate but the stock DTR shock has no damping adjustment so you might end up with the rear being a bit of a pogo stick (I was running the YSS rebound on position 29 out of 30).

The correct way to set up suspension according to a lot of racers is get the correct spring rate first; this is achieved by aiming for 25% static sag (the amount the suspension compresses when you sit on the stationary bike wearing all your usual riding gear, helmet etc.). If you fall down the hole of just adding heavier and heavier fork oil it will make the front end feel mushy and unresponsive. Once you've got the right spring rate, then is the time to start working on damping which is a little more subjective as it depends on where you ride, how fast you go, how you like it to feel etc.

I've made this sound a lot more difficult then it is TBH, the spring rate is something you can do without even riding the bike to get a starting point (good project for a wet day), then a few ride-and-feel sessions and you'll be well on your way. Getting this right can make the bike feel like it really belongs to you and is worth spending time on.

The best place to get a Zeeltronic ignition is direct from the manufacturer, they ask you to send pictures of your connectors so they can custom make one that plugs straight into the CDI connectors on your wiring loom.

I installed one on my '93 4BL with 3NC electrics and by taking power for the CDI from the brown wire (main feed on Yamahas) on the 3-wire servo connector and running a 5-wire servo, it's possible to install it without altering the main loom at all meaning you can revert back to the 3NC CDI/3-wire servo stock setup easily.

Zeeltronic are very helpful and free with advice when you purchase a unit from them as well.

@Calum I've been using Flickr to post pics on here as you say, a bit long-winded but works well

@gary76 Before going down that route I'd recondition your stock M/Cyl as well as the rear caliper and possibly upgrade the rear brake hose to an HEL. I'd hesitate doing any "upgrades" until you've got the stock setup as good as it can possibly be.

The Yambits rear master cylinder repair kit is good but I'd go genuine for the rear caliper pistons and seals as the quality of some of those eBay kits can be a bit patchy. I bought an eBay rear caliper rebuild kit once, cleaned the caliper completely including the seal grooves, put it together and bled it and there was a small dribble of brake fluid on the back of the pad on the piston side. Wiped it off and it came back several times, so I cleaned up the (old) genuine piston/seals and put them back in as I wanted to go for a ride that evening and it cured it, no issues for a couple of months whereupon I bought new genuine ones.

When doing the master cylinder, replace the O-ring and circlip where the plastic reservoir elbow fits with genuine and clean behind this as it gets scabby and blocks the tiny hole inside it. Take your time getting the plastic elbow out as it might be stuck in with more corrosion. New reservoir membrane (Yambits) is also a good shout. HEL rear main hose is a good fit but use the original banjo bolts as these have more threads than the HEL ones. The reservoir hoses on DTRs are now getting on for 30 years old so it's a good idea to change this but you can't just use any old length of rubber hose as the fluid seeps through it at a molecular level (looks like condensation when the rest of the bike is bone dry); Tygon 2375 is what road racers use for non-pressurised brake fluid connections:

https://www.ebay.co.uk/itm/121792758379?_skw=tygon+2375&epid=1747015804&itmmeta=01K6V3C26NA29A5VRZSDJ0J6JH&hash=item1c5b6a026b:g:SCsAAOSwaNBUhc64&itmprp=enc%3AAQAKAAAA8FkggFvd1GGDu0w3yXCmi1dh4qqIOy8SKuCGRxanNoaEisQUPIhYOIwW8GBr4GoH4PB0%2BaVtI0GVNqXnblpufU1h%2BOovMCgzXq41C3VqzwQZGSgufcZicUs7Q5B2TpeIWXhmxh3lUdLEVBaU3TzwxRYHbGRVp9kbh5fF0w8LvAiMyCwfRURy125GMLzRLsLeKdrSjWumBq9qca2gH8WrzInvQAV22nQ%2FX94rYeo7SguAGjDgzcM51M7q0nDNrPW5vyCAnahGEADzpQSd9q3YTRxaxBWs%2FNnGYy13dCM6T786zRUyoXXvnjSG8sbIsrtZOw%3D%3D|tkp%3ABk9SR8KjsOO2Zg

There's nothing really wrong with the stock rear brake on the DTR but like a lot of things on bikes it's very sensitive to how it's maintained. A lot of people neglect the sliding pins which allow the caliper to float when the brake is applied and the rubbers/pins are well worth buying genuine and keeping well greased. Another thing that used to help me is, when going out for a ride with a freshly lubed chain, try and drag the rear brake fairly regularly for about the first 5 or so miles as chain lube goes everywhere including on the disc rotor face nearest the wheel. Doing this burns it off until the chain has flung off the excess and makes the brake noticeably better for the rest of the ride and it's also worth giving the rotor a wipe off with some brake cleaner when you're back home cleaning the bike and re-lubing the chain etc. with it on the workstand. Take out the pads and give them a blast off with brake cleaner regularly as well.

@Calum Alitalia paintwork looks beautiful.

@kubicz The Gillmoto sounds good if it's a like-for-like replacement besides being plastic. Beyond that I wouldn't start putting YZ bodywork on a DTR, I've seen it done and it looked ridiculous to be honest. A modern MX bike tank/seat has a much flatter profile and it just looked out of place. Lots of DTRs have been ruined over the years by people trying to turn them into YZs and you'll never do it, it's not meant for that and the two bikes are very different. The DTR styling is based on the 1988 YZ250 because that's when the DTR was released, just like the Suzuki TS125R looking like a 1989 RM125 with lights. They're in keeping with the era they're from and that makes them beautiful, which is why kids today are dragging DTRs twice their age out of barns and restoring them; thanks to the dumbing down of consumerism, nothing like the 125s of the late80s/early 90s exists any more.

If you're on a limited budget, try and make the stock setup as good as it can possibly be before you start modifying. Do you own a cable oiler and a grease gun? Running genuine clutch and throttle cables, or pattern ones which don't last five minutes? Done an Autolube recon with new seals etc.? Good quality chain and sprockets in good shape? Got a system for cleaning and re-oiling air filters in rotation? D you scrub your bike with degreaser and re-lube everything when cleaning it (see pic; this bike was used off-road regularly), or jet wash it and go for a beer? These are all better uses of your time and resources and will improve the bike a lot more than impressing your mates with a few fancy mods because YZ250.

@kubicz DT200R 3ET alloy swingarm fits straight in as far as I can see, the bosses on this one I bought from Japan were the same size as the DT125R IIRC. It's basically what the DT125R should have had but Yamaha cheapened it with a (much heavier) replica steel swingarm as it was aimed at the youth market; the DT230 lanza also had a steel swingarm as it occupied a similar once-in-a-blue-moon-offroad-riding sector in the Yamaha range back in the day, whereas the DT200R was a slightly more enduro-ready machine (Kawasaki did the same with the KMX125 and a lot of KMX125 guys fit the KMX200 alloy arm).

The 3ET hub is different to the 125, and it runs 520 chain and sprockets; I didn't have any of these to try so I did a bench mock-up using 3ET swingarm, wheel spacers and brake caliper mount mated to a DT125R hub. A 3mm thick M20 washer between the swingarm and brake caliper mount appeared to line up the DT125R 428 sprockets as you can see (these are DT125RE crankcases and gearbox/output shaft). At the time I had no way of fitting this to a bike and trying it but in my view it gave me enough information that I would have been prepared to try it if I'd owned a roadworthy DT125R at the time.

This gets you an alloy swingarm which accepts the stock chain guide/slipper and retains wheel alignment (if the sprockets line up with the stock 125 hub, the wheels line up. Provided you've followed all the steps in the manual regarding setting the swingarm side clearance, the engine boss governs wheel alignment as much as the frame). You also get to carry on using the DT125R 28-1212 swingarm bearing kit and the OEM linkage bushes/seals.

https://huntersmotorcycles.co.uk/product/all-balls-racing-swing-arm-bearings-kit-set-28-1212-1117

I'd say the only drawback to doing this is, aluminium wears differently to steel and the rear of the linkage bearing housing on the 3ET swingarm is particularly prone to this becaue the rear shock mudflap rubs against it every time the suspension moves. Modern KTMs do this as well and some people glue or JB Weld a coin to the affected area. All that mud and chain lube makes for a pretty harsh environment for gluing two pieces of metal together, so onm my 250EXC I used one of those plastic tubes you get in the centre of a reel of sticky labels you buy in the Post Office (last 3 pics). It's a really nice plastic, tough yet flexible, you can cut it with scissors, open it out and use a hole punch to make the holes to zip tie it on. Anything like that will do the job.

There's a guy on DT200R Australia Facebook page called Peter MacDonald who's always rebuilding 200s and selling parts, see if you can get an alloy swingarm off him. Don't forget to ask him to include the DT200R rear wheel spacers and brake caliper mount as well (Oz is full of DT200Rs and shipping to the UK/Europe isn't as expensive as you might think):

https://www.facebook.com/groups/2895277973902854

@gary76 That looks ace! Love the gold rims.

@Mekoz To get the forks apart, a lot of bike shops buzz gun the bottom bolt out. It can help to get an assistant to compress the fork somehow while you do this so the spring pushes harder against the damper rod and makes it even less likely to rotate.

You would then at least have the forks apart which will make it a lot easier to examine the damper rod and see exactly what tool you need to make/buy to securely hold it when re-tightening the bottom bolt after replacing the seals etc.

Lidl impact wrench (just as good as DeWalt etc. from Screwfix) is around £60 plus battery/charger and will come in handy for removing other stuff on the bike like flywheel/primary drive gear/front sprocket nuts etc. (although I'd hesitate to use a buzz gun to re-tighten any of these).

@Calum Thanks bud, yes I still have that tool I made somewhere. Been following this thread and it's puzzling there's no visible means of holding the damper rod like on DTR forks. Possibly someone on a WR125X forum or Facebook page might know?

@YYHpilo It's a lovely piece of machining and I like the O-ring sealing but I'm not sure about omitting the thermostat. As well as regulating coolant temperature, the idea IIRC is to keep the coolant in the actual water jacket around the cylinder and head under higher pressure than the rest of the system to increase its boiling point. Even some competition bikes which don't run thermostats have restrictor plates at the outlet on top of the cylinder head to achieve this. The website says the DTR thermostat is prone to sticking; I've never had that happen on any DTR, including the one I owned for two years/30,000+ miles as a teenager. I did 3 or 4 top end rebuilds on that bike before selling it to a friend who did another few thousand miles before passing his test and getting a bigger bike, and neither of us ever touched the thermostat. I think this happens because people don't replace their coolant often enough or with the correct grade TBH. Or they just use water judging by the rust inside some of the secondhand barrels/heads I've bought in the past.

Also not sure why they'd supply stainless (not to be used in load-bearing applications) bolts for fixing the head to the cylinder; every time you take off the head (possibly to try a different squish shape according to track conditions etc. as the head is marketed as being "for tuners, not for people who don't know what they're doing") the threads in the cylinder will become a little more worn ultimately leading to failure. Incidentally the torque wrench setting for the cylinder/head studs is 13Nm whilst for the cylinder head nuts this is 22Nm (and 28Nm for the barrel nuts); the idea here is, the combination of long thread engagement, low (stud) torque value and being relatively undisturbed ensures long life for the threads in the cylinder and cases which are expensive. Surely supplying their own cylinder studs the correct length isn't beyond the wit of anyone capable of producing something as beautiful as that?

I have to say I don't go much on this guy; the upswept stainless expansion chamber appears to attach directly to the frame (which guarantees it will crack from vibration as the rubber vibration damper isn't present in the photos) and the Supermoto front caliper bracket looks a bit flimsy to me. Considering larger front disc rotor = increased braking forces, I'd want this to be at least as thick/beefy as the caliper mounting lugs on the fork slider. He's clearly good at machining and can in all probability use the two-stroke tuning equations to make an expansion chamber of the correct dimensions but beyond that, he seems to be of the rather dangerous mindset that no-one (including Yamaha) can tell him anything.

@Dougster Even with a pinned powervalve the bike should pull smoothly right through the rev range, just with very little power until it comes on the pipe at 6-7000rpm.Actual backfiring could mean a CDI unit issue but this is quite rare; a lot of midrange running problems on the DTR are caused by a blocked emulsion tube in the carb.

If you've just bought the bike I'd 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 until it just starts fourstroking and won't rev out one day as this controls the mid-throttle fuelling. Make sure the air supply to this 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.

You can also make future carb cleaning easier by bypassing the carb with the carb warmer hoses which come from the cooling system. These are only really needed to stop carb icing in countries with really cold winters but they serve another purpose which is to make the cooling system self-bleeding when refilling, so you don't get air gaps at the top of the cylinder head and unlike a lot of two-strokes you don't need bleed bolts in various places on the cooling system. Whilst you're getting the carb off, you can retain this feature by running a single hose directly from the banjo on the cylinder head to the one on the thermostat housing so you don't have to mess with the cooling system next time. Don't forget to check the coolant level in the radiator after doing this :+1:

@mjmikey Koyo bearings are very good quality but did you order the correct ones for this application? I believe for crankshaft applications they need to have larger clearances to allow for heat expansion; PJME refer to this as "the correct R1 type" but I've also heard it referred to as C3/C4 clearance:

https://www.pjme.co.uk/acatalog/Yamaha-DT125LC-Crankshaft-Main-Bearing-RH-5698.html#SID=103