Mystery problem with '85 DT125LC

[Cynic]

The exhaust will be better fixed but a small leak will not hurt.

I think the explosions you talk of are actually the bike backfiring.

This has 3 causes in a 2 stroke normally. In no particular order.

1 Coke build up. This glows red hot igniting fuel in the exhaust.

2 Ig fault causing the spark to occur incorrectly. Lighting the cylinder charge when the piston is at the wrong point in the cycle.

3 Air leak. On either the carb or exhaust side. Primarily carb side with the crank seals being a common villan.

 

The Yamaha DT also generates its own energy for the cdi. Not battery power. 

My next check would be the coils in the generator. As well as the pick up coil. Heat age or moisture could be causing issues.

[MrMetthew]

Okay guys thanks for the replies. I'm back home, back in the game

First things first I will tonight weld the exhaust shut. 

I aggree the bike is probably backfiring indeed. I doubt an airleak as I've just replaced all the gaskets, glued the crankcase together with yamabond and the problem remained unchanged. Also the reed valve is looking in real good condition to me. 

Coke build up in the exhaust seems possible to me as the exhaust has a reasonable big dent in it. But as this "backfiring" also sometimes occurs when I try to start the engine and everything is cold, I don't see the glowing thing fitting.

"Ig fault causing the spark to occur incorrectly" Is what I'm mainly expecting. But how could I diagnose/fix this? As I understand I can not adjust the timing on my dt125lc. This happens automatically by the ignition coil. I did measure out the resistance of the the both coils in the black cylindrical box as explained in my workshop manual. But the resistances are ok. I will include a picture of the part I am talking about so you guys can be sure what I am talking about. Maybe I should just order a new one of as they are pretty cheap. Or maybe even test with the one of my dt125r. 

Could this be it? A faulty ignition coil? I understand there is some circuitry in this part, maybe a dried out capacitor? I can see them fail that way if there are indeed capacitors in them. What do you guys think? I'm probably just gonna order one to try. The fact that they are pretty cheap and plenty available indicates to me that they indeed fail from time to time.

This is the ignition coil I was talking about.

Also my temperature probe arrived today. Thanks again for the tip Blackhat.

 

[MrMetthew]

Okay, so repairing the exhaust turned out a lot more difficult than I'd hoped for. After closer inspection I figured the last owner tried to repair some leaks at the exhaust inlet with JB weld or something. Anyway the JB weld was seriously deteriorated and was leaking air.

After scraping al of that JB weld off, two large holes and a big crack revealed theirselves . Also the surounding metal was seriously deteriorated and even thinner than it already originally was :p.

To bad I only have a stick welder and a couple of months experience with it. But where there is a will there is a way. So after two full evenings of noobing arround I got to where I find it to be an acceptable fix for now. I used some small plates which I cut and bended to patch up the holes. Will probably have to have another pass after I figure out where my fix leaks, as it probably does, but hey remind you again, I have like no welding experience or skills and only an 100£  cheap china ebay welder. And this was a very difficult weld for me, that goddamn metal tube melts so freaking fast. And again only with my stick welder which doesn't makes it easier, luckily mine goes down to 20 amps.

As far as the real problem goes I am now suspecting either the pickup coil, the cdi or the ignition modulle (this bike has). Will try to test some more to narrow down the problem. Will keep you guys updated.

 

As you can see the damage was pretty extensive..

Again please don't judge me for my stick welding skils 

[Cynic]

For stick welding on exhaust metal its pretty good, the metal exhausts are made from are a long way from top quality.

Why stick welder though? Surely mig would be better for bike welding.

[MrMetthew]

Thanks, and yeah I'm pretty new to welding and hobby metal working in general. I've still got a list longer than my arm of equipment that I want to buy, but yeah I can only spend so much money on this hobby at the moment. What I actually am really wanting at the moment is a TIG welder. 

I actually bought this cheap stick welder off of ebay to repair a small crack in my silencer a couple of months ago. Being it was cheaper than a replacement silencer. Without any experience I took a gamble but it worked out . Now over the last past months I learned to realize a welder is like the hot glue gun of a metal worker. It's such a versatile tool, it's awesome! And certainly in the future I will eventually buy both a MIG and a TIG machine. For now I choose the stick welder because I figured it was still the most versatile and cheap of the three.

[MrMetthew]

Update, the welded exhaust performed better than expected with only one small noticeable leak .

I've made an alluminium bracket to mount the temperature sensor and have installed it. I've been fidelling arround with the the cdi and the wiring harnas while the engine was running and I think that induced a an episode of the bike stuttering. I've checked all the wiring and cleaned all the contacts with contact cleaner and the bike seemed to run like a charm after that (i've cleaned them before to no avail). Although It's impossible to be sure because of the episodal nature of this problem, but I'm now suspecting the wiring around the cdi again or even the cdi itself. 

Anyway tonight I'm gonna replace al the connectors around the cdi and the rest of the ignition system and test ride a bit. If the problem remains I probably going to end up changing all the wiring for new ones. Definitely think I'm getting closer to the problem, but it's a cat and mouse game. Especially with the episodal nature of this problem, very annoying. Anyway gonna keep scratching things from my list till the problem is solved. 

 

 

 

On a side note I've got a triple question about the exhaust system, as you can see in the third picture of my last post with four pictures of my exhaust, I've got quite a significant dent in the exhaust.

1) Do dents like this influence the performance of the bike much? Is it worth the hassle to try to fix (and possibly screw up the exhaust)?                      

 

2) How can I fix this? I was thinking one, welding some studs to the dents and somehow try to pul the dent out. Or two, cutting the exhaust open with my grinder, beating the dent out of the exhaust and than welding it back up again.

 

3) I've heard about gutting an exhaust, which would be the perfect time to try if I indeed did cut the exhaust open to de-dent it. Any thoughts on this? Good, bad performance wise? I've read both ways online.

 

[Cynic]

It will only have a tiny impact on performance, its more complex than a bit of a dent.

I have read books on it and could bore myself to death with it. Its more about chamber volume and overall shape, pulse reflections and such, the dent unless sizeable will have little real effect.The anoraks and perfectionists would say otherwise unless 1/2 a hp and 100rpm matters to you.

I know my DT had several connectors within the loom, on the main feeds where several feeds were needed they had spliced off within the loom. When I rewired my DT they were quite corroded as they were only wrapped in loom tape. That may be something to consider.

[MrMetthew]

Update:

So I checked a lot of the wiring again, removed a lot of the tape from the loom, cleaned al the contacts very thoroughly, but to no avail. First I started her up, worked perfectly, drove for about 2km no problem. Than suddenly she cut out, started running bad again, backfiring. After some minutes suddenly she ran perfectly again . This is soo anoying, it's really driving me crazy.

Anyway found both the wiring diagram for my DT125LC and the wiring diagram for my DT125R. I'm gonna try to run my DT125R with the cdi from the DT125LC. See if the cdi provokes the same problems in the other bike, if so I will have found the culprit! I already figured out the wiring, so I'm pretty sure it's gonna work, unless there is some fundamental difference in the cdi's which after some research I don't think will be the case. Ofcourse the ypvs powervalve servo will not be working, but whatever. 

Question: I've realized the bike I have, DT125LC mk3, is marketed as a 10V bike! I've always ran it with a 12v battery though. Read somewhere that this bice is suposed to have a 12v battery (?). But do you think it's safe to try to transplant the cdi from the "10V" bike to the 12V bike? Never heard of a 10V bike tbh, although I know that the older dt's where 6V.

[Cynic]

Thats the model code. The restricted uk bike was 12w. Yours is 10v.

Like the aircooled DT175 model code is 2k4.

[MrMetthew]

Aah great . Haha I'm gonna guess I'm not the first one to fall in that trap ! Thanks for the info! I am gonna change the cdi's then!

[MrMetthew]

Update:

Okay the cdi switch induced more headscratching than anticipated :p. But got the cdi from dt DTLC almost wired up in the DTR. Just one more wire to figure out but for that I'll have to remove both flywheels and check out the actual wiring of the coils, I'll do that probably tomorrow evening. Pretty anxious to see if the cdi transplant will actually work.

I keep seeing strange things in the wiring diagrams though, although thats probably just me ;). Think I'll maybe post a new topic about that as it is a bit unrelated.

[MrMetthew]

Update: 

Okay the transplanting the cdi didn't work out. Did not get it to run the dt125r. Possibly because first I made a wiring mistake and tried to kickstart the engine, possibly destroying the cdi altogether . Anyway will not know that for sure until I wire it back up again in the dt lc. But that seems too not matter to much as I found after rigoureus online research that my electrical problem is a known problem with the dt lc cdi (according to a very old post on some french forum). Where it momentarelly boggs down or cuts out. So new cdi it is.

Now this is where it got tricky: buying a new cdi :p. I don't now why but the dt lc cdi's seem to be almost impossible to find anywhere. My eye now lays on the "sparker dc-cdi-p2". This is a sort of universal programmable cdi from a company named ignitech and they claim to support the dt125lc with ypvs. The company seems pretty trustworthy with some positive reviews online. I've emailed them to confirm the compatibility with my bike.

There not cheap but at the moment they seem like my only route till an original cdi pops up online, which also are often sold for ridiculous amounts, and also this new cdi is programmable so I will be able to play with the ignition timing in the different rpm ranges which seems like a really fun extra! We'll see what the respond, and in the meantime I will keep an eye open on ebay and such.

[Cynic]

Search speedshop on the forum. 

Its a company that repairs/remanufactures all kinds of motorcycle electronics.

[MrMetthew]

Update:

Thanks for the advice. I checked them out, did a lot of research and mailed some companies. Ended up ordering a custom programmable cdi. A DC-CDI-P2 from the Company ignitech. I did however bought it from the Company Devmoto. That guy, seemed to be the only one on the market who could supply me with (hopefully) correct ignition and ypvs timing maps. Whenever I get it all up and working decently, I will post up a separate topic with a review and some info about this to help future people who have a need for a new dtlc ignition. Because its pretty hard to fînd some good information about this if your New to custom ignitions.

Verstuurd vanaf mijn SM-A510F met Tapatalk

Hopefully this new cdi will solve my problems with the bike. It will certainly give me a powerboost and I will have the possebility to play around and experiment with the timing and ypvs curves a bit. Which if that doesn't sounds fun to you, I don't think we can be friends [emoji14]

Verstuurd vanaf mijn SM-A510F met Tapatalk

[Cynic]

I was looking at something similar on my TDR.

By all accounts with no other mods. Changing the pv timing can give you 6hp (granted im starting with nearly 50).

It also makes the bike a lot livelier. Obviously a little low power is lost and it will be more brutal to ride. Not a bad thing. 

I will dig out the link to the blog. 

[MrMetthew]

Update:

This '85 is turning out being a real royal pain right in the but! I finally installed the new programmable cdi after retarded delivery. And nope, no worky worky. She doesn't start, doesn't even want to start, only a loud bang in the exhaust every kick or five. I am trying to confirm if the new CDI is working correctly, but the only thing I can say at the moment is that it is producing nice strong blue sparks. Offcourse as for the actual timing of the sparks, I can't check that as easily, but I am now starting to think maybe the CDI was not the problem afteral :p...

Assuming I have correct sparks, which it looks like for all of the world, having compression, than I'm starting thinking fuel again in the "fuel-compression-spark" trinity. So I'm starting to look at my carburator once again. Any advice ?

Next steps I'm gonna try, try instal the old (possibly erratic) CDI again, see what that gives. Cleaning out the carb again.

 

[KevtheRev]

Just check that your flywheel key hasn't sheared man . 

[hug]

Hi , i did you manage to sort your bike? This subject piqued my interest as i am trying to get my mates dt125lc3(1hr). Sometimes she goes like stink then suddenly she will not want to pick up at all and just hunts with the occasional backfire. Have changed the piston, new crank seals both sides (they were knackered), put the powervalve back in the right way up ( previous owner had put it back in upside down). New float needle and valve, new pulser coil. If she was just running like a bag of sh*t all the time i would maybe have more of a point of reference but when she goes goes, she goes really well. I will say that sometimes when she is running well on the higher rev range she can be a bit finicky on tickover and not pick up untill i put the choke back on. Also tried another ignition coil. Surely if it was fuel/breathing related it would happen all the time??

[Airhead]

8 hours ago, hug said:

Hi , i did you manage to sort your bike? This subject piqued my interest as i am trying to get my mates dt125lc3(1hr). Sometimes she goes like stink then suddenly she will not want to pick up at all and just hunts with the occasional backfire. Have changed the piston, new crank seals both sides (they were knackered), put the powervalve back in the right way up ( previous owner had put it back in upside down). New float needle and valve, new pulser coil. If she was just running like a bag of sh*t all the time i would maybe have more of a point of reference but when she goes goes, she goes really well. I will say that sometimes when she is running well on the higher rev range she can be a bit finicky on tickover and not pick up untill i put the choke back on. Also tried another ignition coil. Surely if it was fuel/breathing related it would happen all the time??

Have you tried a new sparkplug?

is the HT lead connection to the plug cap secure?

[hug]

17 hours ago, Airhead said:

Have you tried a new sparkplug?

is the HT lead connection to the plug cap secure?

Have tried a plug. New cap, cut the ht lead back a centmetre before fitting the cap. Yesterday had another go. Ran terribly so decided to use a colour tune in order to have a look at the combustion colour and she cleared out and ran good with the colour tune attached. So  thanks for the heads up with the plug as it has been a while since i put a new plug in and it is possible that she is running in such a way that the plugs are getting eaten. The colour tune was quite difficult to read but i would say that she is erring on the lean side rather than the rich. Gonna lift the needle ,fit a new plug and see what occurs. Thanks for the advice Airhead.