Choppy power band
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You shouldn't need to mess with the carb for the expansion chamber, despite how counter intuitive that may be.
I'm likely to say, based off the description, that this sounds like a health issue.
Are you sure that the powervalve is operating as intended, it's not coked up and in need of a good clean.
The airbox is flowing freely, filter not clogged up, oil is lubing correctly, not overly smokey. The spark plug is in good condition, I recommend using an Iridium plug.
The general health of the engine is good?
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He said it used to choke out a bit earlier before he changed the pipe. It was an existing issue from before. The carbs obviously not set right if it has a 240 main jet with the needle set in the middle…
Just need to suss out which iteration of carb it is and set it back to standard…
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@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.
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- 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.
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Check for the right pilot jet: 25 (it makes a difference if it’s wrong, it did to mine)
Put the needle clip on the second lowest groove.
1 & 1/4 turns out for airscrew (you should fix it so you can remove it completely. The o-ring may be perished inside)
These are the settings for that carb…Make sure both jets are totally clear. You should see daylight through both
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Will do. We have an ultrasound bath at work that I will stuff all the parts in.
Perhaps even get a Mikuni TM28 renovation kit from eBay and fix it up well.Thanks!
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@Hark_Ptooie hi bud, first you need to establish exactly which model you have. The 'gizmo' in the speedo you mention is the reed switch which was on the 4BK model, 99-03. Yet the DE03 you claim to have (03-07) didn't have the reed switch but had the green/black wire to earth the CDI. Has the engine been changed? Does yours have electric start? Do you know if it's a DT125R/DT125RE/DT125X/SM?
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It is a 2003 DTR, DE03 which is "mid 99 to 03 models" from what I have read. No electric start, and it has a reed switch - no green/black wire to ground, that came with the 2004 facelift and the RE and X. No?
I find different such specs on different sites, so I suspect it could depend on the market. I am in Sweden, for what it's worth.
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Why does everyone call them a ‘reed’ switch? It’s that thing of hearing it from two different sources makes it right? ( or once on the web!) Like bigger jets = more power (not black plugs
), or are needed in DEPs?
It’s a read switch I found out.Yes I am pedantic!
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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)
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@Hark_Ptooie Just read through this thread again, do I understand correctly you're running an Athena expansion chamber and stock silencer? If so that could be having a negative effect as the stock expansion chamber and silencer are designed to work together on some models. Not precisely the same situation but I once fitted a DEP silencer to my '93 French import with stock expansion chamber/3NC CDI, and it immediately refused to rev beyond 9000rpm. It literally hit a brick wall and I tried it in a few different directions to eliminate the possibility of the prevailing wind that day. This was cured straight away when I refitted the stock silencer; 10,500rpm with no issues. So a Athena expansion/stock silencer combo could potentially also have an unpredictable effect as you're basically messing around with pressure waves, back pressure etc.
IIRC the Athena expansion chamber is the same as a Tecno (exactly the same pipe but unpainted), eventually I ran this with a DEP silencer and stock jetting on the same bike and the performance was basically identical to having a stock expansion/silencer combination. So maybe try a DEP silencer and see if that delivers any improvements. It shouldn't be much louder, just a different exhaust note mainly.
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Hm - you got a point. Yes, I run the stock silencer.
It does rev freely - exactly how far I am not sure at this moment because the tachometer wire needs replacing and I wait for the part, but it is likely beyond 9000. I am quite happy with the peak power, it is that sputtering around some 7000 or so that irks me. Could be related to the silencer, but it was there before I swapped the expansion chamber, though I thought it was because of the restricting.
So, before swap: pulled as well as a 2-stroke 125 cc can down low, had a notable torque increase at 4-6000, then sputtered and choked out over about 7000. Felt like 14 hp, so I figured it was how restricted DTs were.
Now: exactly the same, only it comes again above that sputtering region, and feels a lot like the 23 hp it should be. My old DT's had torque drops about at that region, but didn't sputter and cough.
I don't know the term for it in english, in swedish we say that a two-stroker "four-strokes" during some conditions. On part throttle, much more pronounced in old-time two-stroke mopeds that had no reed valves and such and were terribly tuned. Instead of BWAAAAP, it does some BABWAP BABWAP hurle cough. That is what mine momentarily does in the transition from middle-rev to top-rev.
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@Hark_Ptooie Four-Stroking is usually down to overfueling in that region. You could try lowering the needle clip position to lean up the mixture.
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Oho - thanks for that clue! Will do.
I have noted an unusually pungent exhaust smell from my jacket after driving, and wondered if it may perchance actually run too rich.
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Check your plug bud. If it’s black and sooty then yes…
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Today's quiz: is this the spark plug of a rich, lean or normal mixture? It has only been in use for about 40 miles, but I drove vigorously.
The shiny spot on the far side is where I touched and got the oily soot on my finger. So I'd say it is on the rich side. -
Looks weak to me, if anything…
It would be black if it was rich, in my tentatively humble opinion (because I have made some gaffs lately)
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Don’t know why I didn’t notice it before, but have you put the magnetic speedometer reader (
) switch back in its place? Or moved it completely away?
I think you may still be having troubles with it. It’s right around the rpm’s you’re having trouble at…Read mrShadi’s post (deristrict Yamaha dt 125 r 1997) on his fun with deristricting!…
I’m probably wrong but you never know…
Or did it sputter before you deristricted it?… -
I figured that it was plenty sooty enough considering how short I have driven with it, but I am not certain. Hence asking. Tomorrow I plan to experiment with jetting. I got a new set of main jets.
Yes, it did sputter before but I thought that was part of the restricting. Both my old DT's were unrestricted ('87 and '90) from start. Though I do remember a failure once where the YPVS wouldn't switch to high which felt a lot like this one when it was restricted, minus the sputtering. So I guess it should not have done that?
I don't think the reed switch is a problem, because it reads velocity and cuts at something like 80 km/h, not the rpms. And mine does 110 km/h happily once it gets into the upper power band. Although the front wheel needs truing so I dare not quite find out top speed yet.
My new tachometer cable got stuck at the customs so it will take a while before I know what rpms we are talking about.
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Yeah, I know it cuts in at 60 or so. I just thought it might’ve been doing a ‘strange one’
I’d check the mangled pilot first, replace that. If you have access to a sonic cleaning bath I’d strip down and clean the whole carb. Have you removed and cleaned the emulsion tube? (thing needle sits in) There’s a lot you can do to a carb, all the tiny passages through it with carb cleaner spray or airhose. Even the gasket on the slide body. If the main jet is clear, I’d personally look at the rest of the carb first…
You’ll get there. If I managed to iron out my bike anyone can! …
