DT125r Deristrictions and Mods

[Ryan09]

Modifying a DT 125 (100% certain on 2004 + models)

The first thing to do is to Earth the black cable with the green stripe going through it, you can either remove it from the connector block and earth or (what I did) crimp another piece of cable and add it to the adjacent side of the connector block and then earth it (I used the bottom yoke).

This is probably the most noticeable mod I done to my bike and it’s completely free,

The only other thing I done to my bike was adding a full DEP exhaust system which I found cheapest from here:

http://www.adrenalin...D-2004-581.aspx

I didn’t re-jet after putting this on either as my spark plug was a nice goldy/brown colour although some people have needed to re-jet (depends what climate and altitude, if you live in but the UK I wouldn’t bother).

Some people say about modifying the power valve to give more top end. This didn’t appeal to me as I’m more interested in acceleration and country back roads so I left this alone. If any previous owners had adjusted it and wedged it open you’ll probably find that all they’ve done is tightened the power valve cables so the servo motor won’t be able to turn the PV (power valve) which could have burnt it out or it may have been disconnected completely. To reset this all you need to do is just slacken them off a little bit and follow the image below. (Borrowed from DT125r.net)

Any engine parts such as piston kits, gudgeon pins and cir clips etc for top end and bottom end rebuilds I purchased my parts from: (Prices seemed reasonable to me)

http://www.pjme.co.u...aha_DT125R.html

That’s all the ‘basics’ to get the most noticeable power performance from you DT, Hope it helps.

Theres also loads of other bits for older DT's here: http://www.dtr125.net/

[G Beale]

I bought one of the 1st DT125R's back in the day on an F plate. F820 XAB as I recall. There was a huge washer with a tiny hole in the neck of the down pipe which I removed. Also turned the valve into the fully opened position & locked it in place as the electronic version wasn't readily available in this country then. I fitted a KX250 tail pipe to the standard front pipe & with these few mods it would accelerate on dirt quicker than my mates CR125 & topped out (on the road with standard gearing) at 90mph ! My mate next to me in his car had to tell me my speed as the standard DTR speedo only went up to 80mph as I recall ? Car was indicating 95mph but they all over read so we settled for 90mph genuine speed. There was a huge flat spot at low revs as you can imagine so I just used to cane the nuts off it everywhere I went, problem solved !

I would have another for a bit of fun but I'm 39 now & probably a good 3 stone heavier than I was then.

[mark 88]

ive got a 57 plate dtr, it has the full dep pipe its derestricted and does about 68 mph top but it lacks power at the very bottom is there any way of getting more torque out of the bottom riyan09

[mark 88]

i wonder how the kx 250 tail pipe would go with the dep front pipe ?

[Ryan09]

If you have a full dep pipe why you you go to a kx tail pipe? It would make no difference at all imo.

Are you sure the cable is earthed properly? I know it sounds stupid but its suprising how many people dont get a good earth.

The only proper mods i done to mine was fit the dep full system and earthed the cable behind the speedo. Being a 125 2 stroke there isnt going to be much bottom end power, it needs to be rev'd a lot to get it going.

Even then its hard to keep the momentum going in 6th and it can easily drop off the power.

[mark 88]

yeah wishful thinking on my part lol you can get a 170cc top end by athena for £400 to bolt on but I think it will blow the bottom end. £400 you may as well buy a full 250cc engine and wiring loom and graft if in. its been done

[Ryan09]

Yeah its called the dt 250 lanza lol

[Matthew Duncan]

i thought the lanza was a 230?

[Ryan09]

Obviously i was on about one that had a big bore kit on..

But yeah your right it is....

[sludge]

Had my '89 model off the clock a couple of times (90+mph), usual mods, power valve fully open, full DEP sport etc.

The key for me was to actually LOWER the gearing and increase acceleration, the engine was never able to pull sixth gear until I did so.

I also found the DEP pipes were not much better than standard, even after tinkering with the jetting!

What a great bike, preferred it to the much sportier TZR250!

Now I need another!

[Phil Carlsen]

ive got a 57 plate dtr, it has the full dep pipe its derestricted and does about 68 mph top but it lacks power at the very bottom is there any way of getting more torque out of the bottom riyan09

Hi, I've got the same issue irritating as hell! i'd love to be able to pop the front wheel off the ground, it just doesn't have the bottom

end to do it

[Ryan09]

Its only a 2 stroke 125 so it wont have any bottom end, all the power is up the rev range in the power band. You can still lift the front easily on the throttle in 1st and clutch it up in 2nd, it just takes some practise.

[Lewisarron0808]

Is it a easy job to pin the power vale open as am thinking of doing it to my dtr 2004 plate also getting a dep pipe would I defo need to upjett it or what?

Thanks

[blackhat250]

If you jam open the P.V. ther will be no bottom end power , you"l have to slip the clutch at lights, . P.v were invented for road biast 2 strokes, As old race 2 stroke engines have large exhaust ports,, power is all top end, The YZ125 motoX er is a fine example

. As for jetting ,,,,thats suck it and see = plug chops,,,

[emilz23]

Hello there.

I have my top end - 90kmph, is that green/black wire disabling going to make better top end? Because acceleration till 90kmph is pretty good.

[Karas288]

here's an interesting read about derestricting models 1988-2007 dt's if your interested

http://www.scrimsmustangsclassicyamaharestorations.com/derestricting-the-yamaha-dt125r-1988-through-to-2007.html

[Anditover]

To help clarify what a DT125R can actually do, I have had my bog standard 2002 model dynoed.

It's done about 5,000 miles, I've owned it since it was 13 months old with 1,100 recorded miles.

The only things different to new are the infamous "reed switch" has been moved out of the way, and there is a rattle in the front expansion chamber which could mean that one of the restriction tabs has fallen off.

Sorry, but as a biker of more than 30 years, there are a lot of myths out there. I've owned RD350YPVS, and TDR250, and have owned a KH250 (with S2 350 top end swap) since 1990 so consider myself a BS free zone. My conclusion, from talking to the very nice chaps at Dyno Tech, who run the dyno shop, is that stock is best. We looked at some very interesting curves, including a dep pipe and no filter effort that produced 19bhp at 10,000 rpm, but only 3, yes 3! at 7,000.

Very fast in certain circumstances, unridable the rest of the time. And likely to go pop at any moment.

I wish there was a way to show the graph as an attachment. Suffice to say peak torque was 10.73 ft/lbs dropping to a steady 9.5 ft/ lbs until 9,500 rpm.

Peak rwbhp was a healthy 16.98 at 9,400 rpm.

Mixture was a tad lean at tickover (where no harm could be done), spot on in the mid range, and healthily rich right at the top.

Torque up to 6K was a remarkably steady 8ft/lbs, before rising to the peak at 7k.

Power dropped by about 1hp between 7,250 and 7,750, before rising steadily to a peak at 9,400.

If someone would like me to email them a copy of the curve, I'd be happy to, but for some reason I'm not even been given the option to post it directly.

I've seen 80 mph on the clock, with a gps verified top speed of 82. However, the dyno indicated that both of those may be subject to error, and a more realistic top speed (according to an acceleration through the gears, is about 75mph.

I did take the carb off and check out the "restrictions" at the reed block. Then I measured the diameter of the rubber manifold against the diameter of the exit from the carb. And found that they are the same. My conclusion is that the rubber protrusion is actually aiming the fuel mix at a constant velocity directly at the reeds, adding a slight curve to the route, and that removing them will only increase the internal diameter, reduce the pressure and flow speed, and result in poor running. Either way 17 rwbhp with no mods proves that Yamaha DIDN'T go all out with restricting the induction side of things, and that the exhaust front pipe is probably where most of the inbuilt restrictions are located.

Almost forgot. The best run of the day produced 17.58 bhp, chasing gears (in fifth), and the barometric pressure was about 985mb, in other words pretty awful, and the suggestion was that in normal pressure (1020-30) with cool air, there was the potential for another bhp on top of that. But that's pub talk really, the important thing is that torque was consistent from 4,500rpm to 9,500rpm making for a really strong midrange, and making the peak power easy to get at, as opposed to trying to spit you off if you're in the band and trying to take a corner at the same time, which is not unheard of...

[YAMAHA21]

Has anybody got any info on bypassing the oil injector on a 2007 model, so that you can premix the oil and petrol yourself? Is it hard to do and is it able to be reversed if you change your mind after? Cheers

[finnerz89]

Why do you want to bypass it? 

[YAMAHA21]

6 hours ago, finnerz89 said:

Why do you want to bypass it? 

I've got nearly a full bottle of castrol a747 which you can't use with the injector (I'm trying to get that castrol r smell) and I'd like to be able to use a wider range of oils anyway. However, if it's a hard task then I won't bother. I just thought if it's easy to do I might as well give it a go and see how I like it.