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Old 09-10-2013, 10:39 PM   #91
Inane Cathode OP
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I'm not replacing the carb again, even with a new one. The chances that two completely different bikes with the exact same carb run almost exactly the same is astronomically low. I guess this one could be messed up in the exact same (but undetectable) way but i'd rather not think that at this point.

Haven't found a points ignition for it yet. Bike breaker had 3 very rusty and crusty ignitions, nothing i would want. Oh, one dt360 ignition that looked beautiful but unfortunately the aluminum case yammies have a much larger rotor that doesnt fit under the dt400 case. Theres a points stator on ebay that looks good but its 95 bucks, oof. I think i may have a weirdo stator or something, like a pulser that's dropping out under load or some jazz, even though everything ohms out ok. I'd really like to do a peak voltage test on them but A) I don't have a peak voltage adapter and B) Theres no peak voltage spec for this beast.

At this point i would settle for a bike that runs the same all the time, to hell with ignition curves, i could care less about how the spark duration is so and the spark is fat. I had a dt400 that started by hand and ran like a swiss watch with two sets of points, at least it was testable damn it.

So at this point i think i have a couple of options on the "something weird is going on with this stator" level:

1) Go to the bike breaker and get a complete CDI stator and assume that it was perfectly functional before, and assume that the CDI unit i have is fine. Cheaper, possibly more frustrating option.

2) Go with a points stator, take the inductive ignition and the magic cdi box and throw them right in the garbage, along with any kind of ignition curve, water resistance, etc. More expensive, more final, and a better test of everything else option.

I'd hate to put the new CDI stator in and it runs the same, i still wouldnt know if theres an issue with the CDI box (replaced it once and it ran the same, if that proves anything). Even though it's stock equipment i get the feeling that's kind of a waste of time. At least with points i know for an absolute fact if they are opening, or not, and if the source coil is working, or not. Hmmm.

I think it's worth also mentioning at this time when im testing if its sparking or not, i get a giant blinding spark on the plug but more like every once in a while instead of every time the piston comes up. It might be nothing, or i might be wrong, but it seems intermittent. Would explain why its so hard to start...


EDIT: Looks like ebay decided for me, points stator and flywheel popped up for a 78 dt250. I know the rule of thumb with these is to swap horizontally between years, but parts fishes says the 78 and 75 dt250 use the same flywheel, and i know a 75 250 and 75 400 can interchange ignitions at least plates and rotors, therefor i should be good to go. Watch this space for the inevitable "DAMN IT! IT DOESNT FIT!" post.
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Inane Cathode screwed with this post 09-10-2013 at 10:49 PM
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Old 09-10-2013, 10:52 PM   #92
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Originally Posted by Inane Cathode View Post
Watch this space for the inevitable "DAMN IT! IT DOESNT FIT!" post.
Well at least you said it first so nobody can say they told you so
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Old 09-11-2013, 12:03 AM   #93
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not that it likely means much, particularly for the frustration level you must be going through... but i've found myself checking this thread for updates with anticipation. it's quite a saga, really.

if it were my saga, i can't guarantee it wouldn't have ended by now with a great fiery climax whilst chugging whisky and throwing matches at the bike's open fuel cap. you're a more patient man than i...
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Old 09-11-2013, 01:34 AM   #94
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If you have not fitted a known good or new carb, then you cant eliminate the carb as being your problem. But it sounds more ignition related to me, and very possibly something quite simple.

The most common problem on old CDI systems is insulation breaking down in the source coil when the bike gets hot, and OK again when it cools down. Issues with pulse coils are very rare, but problems in either area will generally mean no spark.

I think you either need to think about looking closely at the carb, and if that checks out ok, and crankcase pressure and compression tests are both good, with reeds sealing properly, find and fit an inductive ignition complete.
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Old 09-11-2013, 11:39 PM   #95
Inane Cathode OP
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Originally Posted by Twin-shocker View Post
If you have not fitted a known good or new carb, then you cant eliminate the carb as being your problem. But it sounds more ignition related to me, and very possibly something quite simple.

The most common problem on old CDI systems is insulation breaking down in the source coil when the bike gets hot, and OK again when it cools down. Issues with pulse coils are very rare, but problems in either area will generally mean no spark.

I think you either need to think about looking closely at the carb, and if that checks out ok, and crankcase pressure and compression tests are both good, with reeds sealing properly, find and fit an inductive ignition complete.
New reeds, seal fine. New crank seals, sealing fine. Compression test 90 psi at this altitude is fine, two carbs absolutely spotless that have the exact same problem has to be impossible. I don't think it's mechanical, i really don't.

I guess the ignition coil could be breaking down or something but its not like its a weak spark. It's either crazy mega fat and blindingly bright, or nothing. When you go to kick these over with the plug out checking spark, it sparks quite rapidly, no? Mine kinda just every once in a while sparks, you know?
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Old 09-12-2013, 12:12 AM   #96
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Your crankcase pressure test checked out ok then? Replacing seals is no guarantee that the cases are not leaking, and its a good idea to check if you have strange problems.

I would suspect in this case its more likely to be ignition related though, and unless you can have the bike properly checked with a scope, the only option here is to substitute known good parts.

Personally I would look for an inductive system, remove the CDI one and fit the inductive.
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Old 09-12-2013, 08:52 AM   #97
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Put the CDI box in the oven at 180 degree's for 20 minutes and see if it changes anything. An old Yamaha mechanic told me this a few months ago.

I had the same kind of symptoms on my MX125C and the CDI box was bad.
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Old 09-12-2013, 09:20 AM   #98
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Quote:
Originally Posted by Scootern29 View Post
Put the CDI box in the oven at 180 degree's for 20 minutes and see if it changes anything. An old Yamaha mechanic told me this a few months ago.

I had the same kind of symptoms on my MX125C and the CDI box was bad.
i gleaned this from the internet a while back. can't remember where. try at your own risk..

"I've read but not tried it myself that dead CDI boxes can be brought back to life by placing in an oven and heating.

"Holy cow, it works. I had 2 units, 1 was completely dead, no spark at all. I put it in the oven at 300* for 20 minutes, connected it and it made spark, kicked it and the bike started but wouldn't rev up. I then took the 2nd unit I had that made spark and didn't rev and baked that one for 20 minutes. connected it and kicked it over. She fired up, and revved just like new. How it works I have NO idea, but it does. "
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Old 09-12-2013, 10:14 AM   #99
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That's interesting stainless. Maybe more than one person on the planet has done this. The OP at this point has nothing to lose. I'd try it, or find another one. RDnutz on Mark's site know's quite a bit about the different CDI interchangability.
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Old 09-12-2013, 11:46 AM   #100
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i gleaned this from the internet a while back. can't remember where. try at your own risk..

"I've read but not tried it myself that dead CDI boxes can be brought back to life by placing in an oven and heating.

"Holy cow, it works. I had 2 units, 1 was completely dead, no spark at all. I put it in the oven at 300* for 20 minutes, connected it and it made spark, kicked it and the bike started but wouldn't rev up. I then took the 2nd unit I had that made spark and didn't rev and baked that one for 20 minutes. connected it and kicked it over. She fired up, and revved just like new. How it works I have NO idea, but it does. "
i've tried it with a dead CDI once. may have worked better if i didn't have a really bad short term memory. put it in the oven, got distracted... a couple hours later i walked into the house wondering what that smell was.

the CDI did work for a short time afterwards, though. bike fired up and was somewhat running OK. can't say i'd trust it for a long term solution... with my luck it'd work great until i was 100 miles from anywhere in the middle of the desert...
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Old 09-12-2013, 12:00 PM   #101
Tim McKittrick
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Originally Posted by Inane Cathode View Post
New reeds, seal fine. New crank seals, sealing fine. Compression test 90 psi at this altitude is fine, two carbs absolutely spotless that have the exact same problem has to be impossible. I don't think it's mechanical, i really don't.

I guess the ignition coil could be breaking down or something but its not like its a weak spark. It's either crazy mega fat and blindingly bright, or nothing. When you go to kick these over with the plug out checking spark, it sparks quite rapidly, no? Mine kinda just every once in a while sparks, you know?
I think I may have mentioned before I had a similar failure with my RS250- no load spark test produced a fat loud spark and it looked like everything was happy, but the bike ran poorly if at all. The affected cylinder would have a wet plug and it looked a ll the world like a fueling issue.

I end up replacing the coils as the connector to them was a fussy proprietary HRC bit and was a bit damaged- the solution was to replace the coils and the entire harness.

It would seem more likely that a coil would produce intermittent results than a CDI box- in my experience the CDI components are almost binary in use, either they work or they don't. If you haven't already swapped out the coil it might be an easy and cheap(ish) thing to try.

At this point we are all amazed you have had the nerve to stick with this as long as you have- and I think we all want to encourage you to not throw in the towel.

Also we all want to know the answer.
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Old 09-12-2013, 08:00 PM   #102
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Just read through this whole thread... WTF! I probably would have burned it by now. Here's hoping mine runs better when the carb comes in...

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Old 09-12-2013, 09:41 PM   #103
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Originally Posted by Tim McKittrick View Post
I think I may have mentioned before I had a similar failure with my RS250- no load spark test produced a fat loud spark and it looked like everything was happy, but the bike ran poorly if at all. The affected cylinder would have a wet plug and it looked a ll the world like a fueling issue.

I end up replacing the coils as the connector to them was a fussy proprietary HRC bit and was a bit damaged- the solution was to replace the coils and the entire harness.

It would seem more likely that a coil would produce intermittent results than a CDI box- in my experience the CDI components are almost binary in use, either they work or they don't. If you haven't already swapped out the coil it might be an easy and cheap(ish) thing to try.

At this point we are all amazed you have had the nerve to stick with this as long as you have- and I think we all want to encourage you to not throw in the towel.

Also we all want to know the answer.
I'm with you, it'd be nice to find out what's going wrong. I'll take a running bike, but if it spontaneously combusts and a little brass plate with what was wrong with the bike engraved on it flies out in the ensuing explosion, i'd be ok with that.

I hope it's a hilariously easy problem that clears it right up, that's the ending this thread needs. "Whoops, plug wire wasn't in all the way!"

Edit: Forgot to actually respond to questions:
Don't know if the crankcase compression is ok, it sucks through the reed cage, doesnt come back out, squirts into the chamber at idle, working enough for me. As i understand it (and in my experience) leaky crankcases are the worst at idle. I could actually run my old ct3 with one reed missing if i revved it enough, hah.

Inductive ignition is just points right? Never heard it called that, new words, yay! I do wonder if i'll have to swap the coil over as well. Would make sense as its just a source coil voltage being broken instead of the cdi shooting a bajillion volts into it. Then again i can fire this coil by poking it with a 6v battery charger so i dont think that's as important.

Actually made my own wiring harness this last week and completely bypassed the bikes harness, didn't change how it behaved at all. Still trouble revving, very flat at high RPM, bizzaro spark test.

I don't feel too bad for saying i'm not going to put my CDI in my oven as an attempt to permanently fix my problems with it, lol. That's pretty wacky though, someone needs to tell yamaha that heating a component will fix it instead of inducing a heat related failure as is suggested in almost every service manual i have ever read

I could see a capacitor failing due to age (they kind of just do that, they puff up and stop being capacitors), i dont see how a thyristor could fail, or how a resistor could fail. I could see how a source coil or pulser coil could fail but the failure mode would be backwards in this case. It should get worse with heat, not better, and in my experience they fail in a binary way. Either working, or not working.

This saga has taught me a few things, though. Had an xl250s with a cracked coil at work that looked at first blush like water intrusion failed the ignition coil and that's why it wasn't sparking. Hit it with 12 volts dragging the leads across the ground, got spark. Checked the pulser coil, bam, no voltage. Replaced the pulser, suddenly we're sparking again and leaking voltage out of every crack and pinhole in the ignition system

I wish i had a fixing buddy with this bike to confirm i'm not retarded, or going crazy, and it is actually this hard to get it working correctly. No worries about giving up so far, i can't let it whoop me. I'll have it running hell or high water (seems like today high water isn't going to be a problem ><). I don't know if anyone saw my thread on the VW with the subaru motor that was acting funny and had me going for a month until i realized i built the harness slightly incorrectly and had the cranking imput voltage for the computer connected to key on voltage, tricking the computer into thinking it was still trying to crank over and obviously not starting, leading to it retarding the timing in two degree steps until the idle circuit took over and jacked the RPM back up. Uhg.


So as a question, what should the spark test look like for this? I kick it over and it's practically a daft punk show of sparks? Or is it more like one or two sparks per kick and that's it?
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Old 09-13-2013, 09:56 AM   #104
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Many years ago I had a very strange problem with a bike, which turned out to be a damaged crankcase gasket, that was ok when the bike was cold, but when it heated up and the cases moved a little, leaked pressure. Seemed very much like seals, and only found it when I stripped the motor completely.

In terms of CDI boxes, you would probably find that the AC coil/CDI units used on Vespa PE200 scooters would work fine. They can be found on Ebay very cheap, and you should be able to wire up pretty easily, as there is only a trigger, feed, and earth to connect.

Never quite been able to work out why people seem to have so much hassle with CDI units, when cheap and easily available units for AC bikes, will in most cases work on other AC bikes, and only need changes to wiring to fit to bikes.
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Old 09-13-2013, 12:00 PM   #105
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Originally Posted by Inane Cathode View Post
New reeds, seal fine. New crank seals, sealing fine. Compression test 90 psi at this altitude is fine, two carbs absolutely spotless that have the exact same problem has to be impossible. I don't think it's mechanical, i really don't.

I guess the ignition coil could be breaking down or something but its not like its a weak spark. It's either crazy mega fat and blindingly bright, or nothing. When you go to kick these over with the plug out checking spark, it sparks quite rapidly, no? Mine kinda just every once in a while sparks, you know?
The results of your compression test concern me!
http://www.marinemechanic.com/New-Folder2/comp-test.htm
At the bottom, read the special note. Not able to pull up any info for the DT400, but your compression sounds low and may be a part of your issue.
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