DT125R FORUM

A 125 cc 2-stroke is capable of delivering over 50 hp. I know a guy that has one in his go-cart - 48 hp at 16500 rpm, and it is not the wildest engine out there.

When some of you say "14 hp sounds right" you should know that those are detuned numbers, the engine has been choked down to produce less power. The DTLC and DTR engines gave 19-23 hp up until the day it was decided that 125 cc bikes should be limited to 10 kW, which happens to be 13.6 hp, give or take some margin.

I have owned three, my first two (1987 & 1990) were fresh and unrestricted and happily did 130 km/h+. My current (2002) was restricted when I bought it and would not do over 80 km/h. Detuning consisted of moving the reed switch away from the speedometer needle, and putting on an expansion chamber without the baffles that were added to the restricted models. It now revs happily to 9500, is every bit as powerful as my old ones. It tops out at 120 km/h because the gearing is too short, but it's fine with me because I only drive around town anyway.

So the bottom line is: no - 14 hp is not "normal" for these engines. They are neutered. In their nominal state you can get 25 hp from a 125 cc 2-stroke without resorting to magic.

The upside is that at 14 hp, they do not wear much. My two first went through several pistons over a couple of years. This third one lived its first 45000 km as restricted and is still on its first piston!

I also have a 2002, a "DE03". I bought an Athena pipe for 150€ which made it rev happily to 9500, then I fixed the reed switch in the speedometer which capped it to 80 km/h. Now it revs out to 120 km/h, should go faster if I swap the front sprocket.

The reed switch should not be removed, because the circuit is needed for some reason. Just pull it out of its socket behind the speedometer (you will find a square thing behind the 80 km/h position) , wrap it in tape, and tuck it in to the side in the housing.

The reed switch should not be confused with the reed valve, which is an entirely different thing.

I did not have to re-jet the carbs, they were fine default.

The Athena pipe did not fit very well, I had to use a bit of violence to get the screws in their sockets, but once there it had no issues.

I hoon around town on an unrestricted DTR and it uses less than 5 L/100 km.

Great, thanks!

The Dominator looked terriffic though.

Okies, very good - thanks a lot!

3AJ = 88-90 Ténéré? Could it be that they had the same Showa 41 mm as the rest of the 84-89 XT600's? Because it may be easier to find.

I actually had a 1990 XT600E. It was a pig. Not recommended.

Quick answer since I have a 2002 as well (DE03):
1 - Swap expansion chamber (the blobby exhaust bit) to an Athena or something. This was the big thing.
2 - Pull out the reed switch from behind the speedometer. Do not disconnect it or cut any wires, just wrap it in tape and stuff it aside. It detects when the needle gets to ca 80 km/h and cuts ignition there.

That should be it, your bike should now rev freely to 9500 rpm and do 120+ km/h.

Also: ensure that the YPVS is working as it should. Remove the round cap on the left side and turn on ignition. It should rotate back and forth a little, then settle such that the groove on top is right over the hole.

Explanation on that last bit: The YPVS switches the port timing between low revs (more torque) and high revs (more power) as the revs increase and decrease. Variable valve timing, you could say.

When you turn on the ignition it rotates back and forth (flips between low and high mode) to scrape off soot. The resting position should be over the hole because they used that hole to fixate the valve in low-revs mode (and turning off or removing the solenoid actuating it) when they restricted bikes, but I believe they stopped using that method after 1998 or something. If it does not rotate back and forth with an audible "bzzt-bzzt" when ignition turns on, your YPVS is not working and stuck in either of the modes.

As for the inlet manifold I did a thorough CFD analysis and found that you cannot improve it. Leave the plastic bottle on, it smoothens out the gas flow. The carburetor should not need anything either, it is likely a Mikuni TM28SS with a 240 µm main jet, which is perfect.

I know little about suspension.

How do I make my front a bit stiffer? I am like 100 kg fully clad and the suspension is a bit too much enduro, too little motocross for my taste.

Is it like I heard about some other bike that there is only one spring in one leg, so you stuff a second in the other for twice the stiffness?

Well, I suppose they saved several pounds of cost for the servo for a while...

Oh - you mean that they removed the servo and put the valve in "low" as some means of restricting the bike?

What a terrible crime against the spirits of engineering! >_<

Gorgeous color scheme!

@Vcelicka said in Powervalve fully open:

Hi guys, i was just wondering if any of you run fully open powervalve? I dont know if its better than normal ...... if you do can you describe difference is there any power boost or its only get bigger MPG ?

You seem confused.
The power valve adjusts the port timing such that it better matches low and high revs.

If you mess with it, you just make it stick to either low or high rev setting. In the former you lose peak power, in the latter you make the lower rpms suck.

It is not a manual crane to twist for some kind of optimization, it is automatic.

Found it! 45-50 µm on the diameter. Wear limit 100 µm

25 µm per side is hella tight.

I have them on PDF. Where do I put them so others can find them here?

More specifically, 1988 3BN-ME and 1989 2YY-AE1.

Okies, thanks!

@erion1 said in engine piston check:

@Hark_Ptooie yeah its the diameter of the piston, the standard is 56.00mm if you get a bigger one you need to rebore the cylinder so that piston can fit

OK, thanks - and what would the appropriate size of the bore be? +0.1 mm? +0.5 mm? +50 µm?

I may need to swap piston in the not too distant future, and motorcycle workshops are few and far between where I live. So it may be that I will need to turn to some car shop and tell them how it should be, because they are not used to this type of engine.

I have heard anecdotes where a faulty boat or moped engines were cured by drilling a hole in the fuel cap.

Just a quick dumb question: when you order pistons they have a size listed such as ø56.5 mm.

Is that the diameter of the piston, or the cylinder hole it is supposed to go in?

If it is the diameter of the piston as I suspect, how much larger should the cylinder diameter be?

My 2002 DE03 DTR has gone 45000 km, supposedly on the first piston and rings according to the former owner.

Sure, it was restricted for 21 years, and wear is quite exponential to load - but still... I would expect the bike to be on its third piston or some such by now. I have had DT's seize on me at full tilt before, so I was under the impression that a piston and rings do not really live more than like 15000 km or so?

But when I put the endoscope in the exhaust port of this one, it looked fine. Had horizontal lines on the cylinder, even.

The cylinder sucks in air at intervals, leading to a pulsation in the inlet airflow. That bottle is a buffer that smooths out the pulsations.

The piston goes up, sucking in air through the carburetor which is (and should be) the main restriction in the intake. This causes a low pressure on the engine side of the carburetor, including that bottle.

When the piston goes down and closes the intake port, there would be a rather sudden pulse where air rush in through the carburetor and meets a dead end. Pressure rises, gas stops throughout the manifold, including the carburetor. Then when the port opens and the piston sucks in gas again, it would have to accelerate the gas mass from zero.

With the bottle, the volume of that closed-off manifold is artificially increased, so it takes longer for the gas to fill the cavity and come to a stop. Hence it keeps moving, supposedly until the port opens again and the piston suck in gas again.

The effect should be that there is a more constant flow of air through the carburetor, which likely improves the atomization or decrease condensation or whatever.

4-stroke engines usually tune the intake manifold runner lengths such that the air pulsations should cause the "pillar of gas" that comes at the intake valve at high speed should "ram" through it when it opens. While the mass of the air is small and it is very elastic, the velocities are substantial so it counts. I wager this is sort of the same thing.

In 4-stroke engines it is also not uncommon that the intake runner lengths vary with engine speed. I know my car has a valve that opens and shorten the length above some rpms. My Honda Fireblade also had something like it in the airbox.

I found a company in Germany, for those who are interested:
https://alphatechnik.de/

They seem to sell kits, if my german is anything to go by:
https://alphatechnik.de/leistungssteigerung-fuer-yamaha-dt125r-de03/05-de03-k19

You can search for other models as well:
https://alphatechnik.de/leistungssteigerung-fuer-yamaha-dt125r-4bl/05-4bl-k19

And I found the company through this interesting site:
http://dtpedia.de/mediawiki/index.php/Hauptseite

Where they for example write about the quite interesting reed contact:
http://dtpedia.de/mediawiki/index.php/Reed_Kontakt