DT125R FORUM

Well, nuts and bolts are standard across industry, so you don't need some special kit.

I go to my local hardware store where they have open packs of all sorts and sizes. You put them in a bag and weigh the bag and pay per kilo.

Most screws I have swapped so far have been M6x12 or M6x20 with some odd M5 and M8 here and there. I usually get flanged screws and skip the washers.

I do so love screws. Then again, I am a mechanical engineer so the fetishism comes with the profession. If you want to talk screws joints for a couple of hours, I'm your guy.

Made a template from cardboard, cut one-ply prepreg into shape, bent it over the back of a folder and cured in kitchen oven, drilled screw holes, done.

Sorry for commenting on a 3 year old thread, but I have worked a little with balance shafts on humongous machinery and wanted to post a little for posterior knowledge.

A single balance shaft on a single-cylinder doesn't really do much. It mostly just phase-shifts the vibrations so they go in another direction. Back and forth instead of up and down, like.

Dual balance shafts is another thing. They cooperate in the right direction, and counteract each other in between - and if you put them slightly offset each other vertically they can even mitigate some rotational moment.

But balance shafts also have another interesting effect - they can reduce the bearing load. There are actually two different optimums - one which gives the least vibrations but a bit higher bearing load, and another that is the opposite.

So when I see that small engines have a single balance shaft, I usually assume it is to allow it to manage high revs by means of lowering bearing forces rather than reducing unpleasant vibrations.

There was a bike back in the 90's(?) - some european single-cylinder 2-stroke 250 cc enduro (cross?) - that had a balance shaft and managed to output 70+ hp when all the balance-shaft-less big-brand competitors only gave 55 hp or so. Because it revved to like 12000 when the others quit at 9000 or whatever.

I recently derestricted my 2003 DE03 DTR, and it amounted to pulling out and isolating the reed switch in the speedometer with a fat wad of electrical tape, then swapping the exhaust resonator for a tuning part.

Among other tricks I have read is to cut off the rubber lips on the intake manifold. Or whatever that gizmo is named, the one that you plug the carburetor into.

So I bought a brand new one from eBay, because I figured I would experiment on one and have a spare if I b0rked something.

However - when I received it and had a good look, it did not strike me as that anything about it would restrict gas flow. It is a circular passage that bends and transforms into a square "beak" that enters the reed valve cage snugly. The cross-section area does not have any constrictions to speak of.

So I did what any normal person would of course do and spent a few evenings modeling it in 3D and running CFD analyses on it, reed cage and all. Testing various modifications.

And lo: I could not improve it. Removing the rubber lips, rounding off edges, carving away material this place and that - all resulted in something like a 3% flow loss.

The only positive results I got was from slighly rounding out the insides of the square front of the passage. That gained me something like 0.2% mass flow. You would not even be able to measure it on a dyno.

So leave it as it is. It is not a restriction - on the contrary it improves the flow into the reed cage by removing turbulence right before it.

I'm kind of impressed how right they got it back in the 80's, with no CFD software to speak of.

Just for closure I want to add that the sputtering went away by itself after a hundred kilometers or so.

I'm guessing old grime in the vaporizer that recent use dissolved. It had been standing in a garage for quite some time when I bought it.

Bike now goes like my old 1990 did, only even more cheerful what with the Athena expansion chamber.

Because it is a reed switch.
https://en.wikipedia.org/wiki/Reed_switch
It is a sensor that detects the speedometer needle getting to a certain position. It is disabled by gently lifting it away from that spot, so it detects nothing.

Not to be confused with the reed valve, which is an entirely different thing. That is a cage with reeds in it. https://en.wikipedia.org/wiki/Reed_valve

I am assuming they called the blades "reeds" because they are similar to the blade in the mouthpiece of some instruments. https://en.wikipedia.org/wiki/Reed_(mouthpiece)

@SpookDog said in Choppy power band:

Does it have a yellow plastic ring that fits under the slide spring? Also a plastic tube that fits into the pilot jet tube and around the main jet?…

Yep, and yep.

Airbox is clean, new filter, oil seems right (some blue smoke on choke start, disappears when running, my jacket smells of 2-stroke when I stop...). The intake manifold rubber thingy is pristine and uncut. *) I can't really turn out the air screw more than 1½ rotations, because then it gets stuck and the groove is damaged.

Before the new expansion chamber, there was no noticeable difference between the old and new spark plug.

However, now when it's mentioned - it did do that sputtering before the expansion chamber swap as well. I just assumed that it was part of the restricting. Since it occurred right before the nonexistant upper power band.

When I looked at the powervalve position and felt inside the exhaust port, it was rough and sooty. So I suspect the YPVS.

• • Some apparently attempt to tune these bikes by cutting off the rubber nose thingy that goes into the reed valve. I actually spent my weekend doing a CFD analysis on this and found out that it is useless. Removing that square funnel drops the mass airflow with about 2%. Trimming its edges though to make it slightly more oval can increase the airflow with about 1%. Tops. Then again, I am a dilettant at this.

I put an Athena expansion chamber on my 2003 DE03 DTR and while it works great the fitment was terrible.

Up front I had no problems putting it on the exhaust port.

The first screw bracket was about 5 mm too far back - no real problem because the console it is screwed to is itself held by a screw, so I could just loosen that up and get the threads in. Tightened it last.

Then the second screw bracket - the rubber lined hole - was 10 mm too far back. Ended up removing the metal inserts and apply physical strain on the rubber to get the threads to take.

Then the rear end of the pipe was more than 10 mm too long, so when I attached the (stock, original) silencer I basically had to jam the pipes into each other. The Athena was slightly thinner so it went inside the silencer pipe with some force. But I could only fasten the rear silencer screw, the front is way too off.

Since the Athena pipe is thinner than stock, the rubber tube thingy that is supposed to seal the joint did not tuck on snugly. Put a hose clamp around it.

I plan to remedy things with a hacksaw and file in the near future, but at least the exhaust system works well. A bit zingy sound, not overly loud, and it revs to 9700.

The paint job was not too impressive either. It looks good, but the paint layer seems thin. I may add a coat of something if I only manage to figure out what type of paint to use.

Expansion chamber S410485300007:
https://www.athena.eu/en-se/expansive-muffler-for-discharge-P27835.htm

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

Thanks!

I'll be ordering a small piece of CFRP prepreg to form a chain guard next. I found a place that sell "test specimens" that just so happen to be exactly the size I need, for about £6.

Yeah, I suppose. I went back to uni when I was 30 and had a CBR929RR. The rest were all like 19 year old and didn't even meet my gaze, least of all talked to me. And one day I found a hole in the saddle.

Former owner did creatively rationalize away some stuff on my bike, such as the passenger footrests.

I did not find any DTR on eBay, but new ones for the LC. So I got those, and realized that the difference is that the brackets had screw hole c/c 80 mm while the R has 100 mm.

Or "should have" I suppose. Mine turned out to be 104 mm starboard, 96 mm port side...

So I did some FEA optimization with the structural integrity constraint "withstand daughter climbing on with her full weight on the tip of one peg" and had them laser cut and welded. Then I painted them and moved over the pegs from th LC set.

Fit snug, and look fine, I think.

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.

Great information, thank you!

I figure I should spend the snowy months getting new rims and spokes and putting them on the old hubs. I have one of these I'll convert into a truing rig:

Original parts are listed at hilarious prices, most of the time Out Of Stock and generally not available, I'm pretty sure that if I would ask for DT wheels at the capital they would happily order them from Japan and charge me a month's salary or something. So it is eBay or generic parts.

I find it outrageous that nipples do not use standard dimensions like 5, 6, 8 mm. What were they thinking? I mean, it's not even imperial they're using. Madness.

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! >_<

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 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.

1997:

Hilarious thing, but it developed a plenitude of electrical problems that I was to inept to manage, so I sold it cheap. Regrets.