Choppy power band
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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!
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I should report that the last weeks the sputtering has improved significantly without me doing much of anything.
Well, I did twist the air screw a bit, and my new main jets arrived and lie here on the table, and I am waiting for a Mikuni rebuild kit - but in the meantime the bike decided to behave better. It is weird.
It still hesitates a bit around 7000 before coming into force until 9500, and I can probably improve that with a bit of tweaking, but it is largely acceptable as is. And actually it feels like a bit more than 23 hp, comparing the seat-of-the-pants feel with my old '87 and '90 DT's. They would hesitate at 5000, then come alive at 6000 and lose breath at 8500.
I can't imagine that a 2003 bike that seems unchanged in design since 1989 would have anything like modern self-learning ECU tricks, right? I just assume the CDI box would be rather primitive. Right? Right...?
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This post is deleted!
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@Hark_Ptooie no sensors around to learn from...
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@Mr-Shadi had the rev limiter cutting in at around 6000rpm in all the gears at one point. I know it’s not what you want to hear but it’s got to be worth checking, no?
Just try reconnecting it to see if it revs freely in the lower gears. If it’s the same then At least you can cross it off completely. Added bonus is I’ll shut up about the read switch 🤩 -
The engine pulls freely to redline on all gears. It was just around 7000 where it sputtered and coughed. Still does, but far less.
If there are no sensors, then the only remaining reason why it would run better now than before would be that I filled up with new gas. I suppose the old stuff had been sitting in the tank for a long time when I bought the bike...
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Could there be a chance that it’s affecting it partially? It’s not that big of a job to check and cross it off completely. Remember, the added bonus is I’ll shut up
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The reed switch is thickly wrapped in electrical tape and put far away from everything, as far as I know.
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