Mystery problem with '85 DT125LC

[MrMetthew]

Hey guys,

I am desperate trying to debug some strange problem I have with my freshly restored DT125LC (85 ypvs model with active powervalve), and wanted to ask you guys for your expertise.

First some context. I bought her as a wreck at the start of this year, she was in really bad state hadn't been started in what the seller said was over ten years and standing outside in the rain for more than a year atleast (btw who does this?). So full restoration project, fully stripped her, redone frame, all new plastics, lot of new electrical connections, kilos of sanding paper, couple cans of paint, case splitted, all new bearings, rebored cilinder, new piston, carb cleanes, ...

Anyway here I had a shining, almost like new, 85 dt125lc. Started really good, drove insanely nice for a 125, powerwheelie in first, unbelievable what these old machines can do. But Than a problem started to occur.

Sometimes, about 5-10% of the time maybe, she like went in tho this problem mode. just from one second to the next she went bad. The throttle response was gone, the engine turned really inconsistend just like it was missing some of the ignitions, power gone couldn't even pull me, and there were loud bangs, explosions in the exhaust. When she had an episode like this where the motor was I think missing ignitions or something sometimes she would stall and when that happened you would not be able to start her most of the time until she was cooled down. 

But the big thing was these episodes just stopped as quickly as they came. One moment when the bike was running PERFECT and than just BAM, she entered an "episode" and started doing all these things and then after a while, just BAM again she runned PERFECTLY and you could continue your dirttrail. The episodes really were not something subtile, the bike really went from 100% to about 5% to again a full 100%.

But because this happend on such unpredictable times and went away so quickly (after 15 minutes mostly) and the bike drove perfect for about 90-95% of the time I ended up riding er like this for maybe 400miles (after rebuild). 

 

But then recently while she was having a heavy one of these episodes, suddenly she stalled, no more compression, checked the sparkplug => molten metal ):. Opened her up to find a big old hole in the piston (check for pictures).

 

WHAT I MAYBE SUSPECT:

First I thought maybe bad adjusted carb, but the sparkplug color was relatively good I remember. But important to note when she was having an episode the sparkplug was sticky black. At the moment I am suspecting a bad electrical connection to the sparkplug as a lot of other connections where bad on the bike, although I cleaned a lot of the connections but maybe not all of them. I have experience with the sudden coming and going nature of problems with bad connections and this fits the bill. Missing ignitions seems to be the problem in my eyes at the moment because it would explain the bikes misfiring and unburned fuel in the exhaust to explode. However I don't find any electrical bad connection and I have all the spark wiring laying on my bench and they all look fine.

 

WHAT I'VE DONE ALREADY

°Got the engine out

°splitted the crankcase, cleaned out all the metal 

°Ordered new piston (the bore is not damaged, thank for that)

°Also ordered new sparkplug cable and new sparkplug connector

 

==> I am rebuilding the engine, but I don't know why the piston blew and why the bike had this strange problem (I assume they are connected??). And I would prefer to get to the bottem of this so I dont just blow the engine again :), rebuilding and running in is kind of annoying to keep doing  

 

I've put a LOT of time, effort, money and love in this bike, went to the insanely bullshit bureaucratic world of getting papers for this bike (bought it without them => last time I do this btw). And would really appreciate your guys help! 

THANKS IN ADVANCE  !

 

[finnerz89]

Running lean, so too hot. 'Episodes' were probably the engine seizing. Excessive heat has then finally holed the piston and killed the engine

I'm surprised the bore isn't damaged

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[blackhat250]

Yip ,detonation, is usually the cause,  [ google it its in depth ]  , fit a toura tech  temp sensor, [fit to spark plug, ]  if she runs into the 450f, pullover,  ,, cool down limp home and re-jet, and check timing,,

http://www.ebay.co.uk/itm/Trail-Tech-TTO-Temperature-Meter-Orange-Digital-Gauge-14mm-Spark-Plug-Sensor-ET3-/231928631316?hash=item3600061014:g:Q8EAAOxyHE5RoMO8

[KevtheRev]

Lovely clean spark plug + hole in piston = LEAN ,

[MrMetthew]

Thanks guys! Problem not so mysterious afterall :p. I ordered that temperature probe btw, very handy piece of kit !

Will report back after rebuild is completed

[MrMetthew]

Okay guys, parts finally arrivered. Rebuilding as we speak. Whilst I had the case apart I gave everything a good clean, polished it all up and paited it. Was not going to do this but I had the time waiting on my parts, so why not.

I have an question. Anyone knows how this headgasket goes on? It also fits upside down. Its an Athena one, couldn't source a genuine one.

[slice]

They are usually marked in some way TOP and FRONT or "T" "F" which gives you a pointer to were they should go, do you have the old one? That might point you in the right direction.

[KirriePete]

They are usually marked in some way TOP and FRONT or "T" "F" which gives you a pointer to were they should go, do you have the old one? That might point you in the right direction.

Is that why the wife's knickers used to be marked C&A?

[Campaman]

Looking at the recesses on the head and the raised area's on the gasket I would say that the face of the gasket you have showing upwards should go face down on the head, unless the gasket is exactly the same on the other side.

[MrMetthew]

Nope recesses are the same both sides. I've scoured the internet unable to find an answer. And also no the gasket itself doesn't give it away, nothing written or stamped on it. I've just ended up modelling it after the dt125r head gasket, figured they're close enough.

ALSO old main jet when she blew was a 200. Is it safe to now start with a 210 as a starting point and go from there? Or how much should I increase the main jet for a safe starting point? A 210 is all that I've got right here and now.

[Cynic]

200..... fk me. We were putting 250's in some of the 'tuned' bikes we 'improved'. Our favourite was a take the powervalve assy from the left cyl of a 350 and use a choke cable to opperate it. Low for wheelies, high for speed. Long time ago now though.

No wonder she blew at 200. Was that a factory setting. If yours was factory full power the carb will have a different needle and emulsion tube etc which will make it lower but i thought the full power bikes used 220's. Dont quote me though.

Anyway if there is no specific marking on the head gasket and is identical from either side fit as you please.

[MrMetthew]

Thanks for the info. Mine actually has a working powervalve servo, a 5wire one stamped with "27Feb85DC", I don't know if this is an original one can't find anything about it. Also I don't know if the carb is factory full power. I did ran the serial of the bike with the importer and they confirmed this was an original Belgian import bike. Judging from oure current rules I don't think the bike was factory full power, but geez back in '85 who knows. Any idea how I can distinguish?

Anyway still trying to figure out which would be a good starting main jet, 220-230 maybe then? I don't have much experience in rejetting carbs and it seems like it's an expensive learning curve ;p. Don't want to blow her again.

[Cynic]

Using a very un technical assumption my totally factory stock tdr250 runs 210 mains.

So its worth seeing what the 210 does. Esp as you have one.

Then go from there. If the 210 makes it run like shit and chokes then it may be more ig based.

Do you know the ethanol content of your fuel. Here in the uk there is regular unleaded that can be 10% ethanol. That means bigger jets as the ethanol increaces the oxygen content of the charge, (leaner) AND reduces the relative petrol quantity by the same %. Leaner still.

Also chemically attacking any brass and some of your seals too as well as holding water.

Why i use premium fuel here which so far is ethanol free and you can tell the difference.

 

[MrMetthew]

No I actually didn't know that there was ethanol in my fuel, and although biofuels haven't really broken through where I am, a quick google did reveal ethanol in a lot of fuel where I live. Hard to find the exact %, I'm gonna ask the gasstation attendant next time I go there, I am quite interrested in this.

Anyway I've put the 210 in and put the needle to the most rich position, think I'm gonna try to start her like that

[MrMetthew]

I wanted to ask you guys this because there is a lot of disagreement about this online, so it seems. Whats your break-in procedure? Specifically in my case with only new piston/rings. But while we're on the subject, also with a new bore.

I read a lot of contraposing things online, otherwise I would not ask. I personally always do 2 or three heat cycles warming up, cooling down the engine without riding and than start riding it easy, under 6k rpm for about 100-125miles. Is this to much on the safe side maybe? I ask because it's kinda a pain in the ass

[Cynic]

Just last year i re did the top end on my tdr due to a piston ring peg not stopping the ring from rotating and clipping the port.

After the rebuild she got a gentle see what falls off trip to my mums. All told around 10miles round trip.

Then ridden normally. Two weeks later i set off on a 600 mile weekend away at squires.

Running in is tripe. If it aint broke in the first 20 to 30 miles it wont.

 

[blackhat250]

When upsizing main jets,  its best to set needle clip in the middle,  then you  can go up or down to fine tune,

get a temp sensor, [spark plug type, ]  no more seizures,.  pull over when shes above 450*f

[MrMetthew]

@Cynic Thanks, really appreciate your opinion. Seen you around on countless DT topics.

@blackhat250 Thanks for the tip, I actually did order one! Waiting for it to arrive from the USA, ordered a trailtech. Gonna start using this after all my rebuilds, carb adjustments and for heavy offroading.

[MrMetthew]

UPDATE:     !!!

Rebuild fully completed. Bike starts very smooth. But the strange problem remains! Sometimes she just cuts out, missing ignitions, not all of them though just some. Also explosions of unburned fuel in the exhaust. And than suddenly she runs perfect again. Sparkplug temp sensor hasn't arrived yet, but I'm monitoring cylinder temp with infrared temp probe, hasn't gone above 75°C/167°F, also not during her running problems. 

What I've tried so far at the moment is try to monitor the sparking in the sparkplug. For this I wrapped an electrical wire a couple of times around the sparkplug lead/wire and connected that wire to my oscilloscope. When a spark forms, current flows through the sparkplug lead/wire, which in turn induces a voltage/current in the wrapped wire, which I could monitor on my oscilloscope quite clearly. This way I could monitor the spark timing.

The idea was to see if she was missing sparks when she started to bogg down or if the problem is something else, as this would greatly help to nail down the actual problem.

Offcourse Murphy, and she refused to bogg down when my test set-up was in place and she ran perfectly the whole time. I think I possibly I saw some missing sparks when she started to bogg down for a very little while, but I can't be sure and totally wasn't able to capture it on camera for you guys, as intended. I would tried more but I had to capture a plane, as I am now currently on vacation in the sunny portugal, bbqueing and sipping wine till next monday ^^.

Further work on the motorcycle will have to wait till next monday, but I can order parts from here if you guys have any ideas.

 

As a last note I did note a lack of power in my short test rides, it was like the engine had problems gitting into higher rpm, even in neutral. Offcourse this can be the not yet fully correctly adjusted carb. Also there is a rather small, but significant airleak in the pipe, right at the outlet of the engine I discovered. I'll have to weld this but I would be most surprised if that had anything to do with my still remaining problems.

Ideas ?

 

[finnerz89]

I'd definitely sort the hole in the exhaust first, it's an easy fix and may well have an effect on the running

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