Some DT400 Jetting Stuff

Discussion in '2 smokers' started by Inane Cathode, Aug 20, 2013.

  1. anotherguy

    anotherguy Long timer

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    The timing spec is for good fuel.......that is fuel not sold at pumps anymore. Retarding the timing a bit will help with the pinging. Or run race fuel.

    And plug color is just fine to confirm jetting. It's just a different color.

    [​IMG]
    #81
  2. Tim McKittrick

    Tim McKittrick Long timer

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    It's easy enough to pop the head off and have a look: detonation damage will look like some has been sandblasting the edges of the piston and the underside of the combustion chamber. First the ring lands will get pounded closed and trap the rings, causing lost compression and blow-by, and eventually the piston will get flattened and holed. Excess carbon buildup can also cause detonation as a glowing chunk of hot carbon acts as a glowplug and fires off the mix too soon, so (If you haven't already) removing the head would allow you to de-carbon things at the same time. It's possible you coked things up when running it with the faulty over-rich carburetor.

    A rattling pipe can be ruled in or out during this as well, since the head pipe has to be removed (I think) to get the head off. you should be able to bonk it an find loose baffles that could be responsible for the noise.

    And the horn thing is just odd.... if the horn works at all the only explanation I can come up with is you are providing a partial ground when it is energized- but how that would be siphoning of current from the ignition system is a mystery. My DT175 uses it's battery as a buffer for the charging system, but it has a points type ignition. It'll run without a battery but nothing else will work, and it'll fry all the bulbs. Is your battery in decent condition?

    I have found it hard to read plugs with modern unleaded gas, and it is often easier to work down from over-rich. You could tune with race fuel, but a switch back to pump gas might require additional tweaking.
    #82
  3. Inane Cathode

    Inane Cathode Cheated Anion

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    Went out for an offroad ride today. It's not rideable, the running thing comes back with vengance while offroad. Extremely flat wide open throttle, chokey aweful running everywhere else. Doesnt change with jetting, didnt change with new plug, using the horn button acts like a rev limiter with higher RPM, at low rpm it doesnt do anything, but it does sound weird, kinda poppy instead of honkey.

    No battery, all the bulbs are unplugged/burned out. Haven't gotten that far. The horn thing is friggen weird. It's supposed to be a separate circuit, i have no idea what the horn circuit would have anything to do with the ignition circuit.

    The only thing i can think of is when you hit the horn button you're actually providing a ground for the source coil. At higher RPM you've got incoming power from the lighting coil hitting the incoming power that's trying to find a ground. Without a voltage potential, there's no current flow and no work gets done. I don't know, it's an idea.

    Well, i really did think it was the carb, i didnt do any other work at the time so i dont know why it seemed to change so much. I'm getting the funny feeling i'll be building my own wiring harness for this thing ><

    I'll do a little more intensive voltage drop tests. The horn thing is making me think i've got some goofy grounding problems going on in there. I'll collate the various diagrams i have and see if i can isolate what's going on here. Once again i feel like i'm sniffing down the right trail.
    #83
  4. Inane Cathode

    Inane Cathode Cheated Anion

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    Well, back to running shitty. Runs exactly the same with everything except the ignition plugged in. Pulls for just a second at low rpm then blubbers and four strokes it's way up to what feels like a rev limiter where it feels very flat.

    Every coil ohms out as it should, runs the same with two different CDIs. Same with two different carbs. Made my own harness from the stator to the cdi to the coil, runs the same. Timing light says it bounces around quite a bit going up to the 'rev limit' then it goes full retard at high RPM, used two separate timing lights, both act the same. Once the bike is very warmed up it runs pretty good but pings quite a bit. Does not run worth a damn offroad. Hardly pulls, won't climb anything, four strokes very badly, completely unrideable.

    I'm completely out of things to try on this bike, literally. Every single system has been checked, rechecked, and replaced with no change in how it runs except very minor differences in how it pulls. Totally out of ideas. I'm just about ready to throw this whole bike in the god damn garbage.

    I'll guess i'll waste even more money and convert the thing to points. I'm sure it will run exactly the same anyway.
    #84
  5. stainlesscycle

    stainlesscycle Long timer

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    any chance we can see the plug? i assume a reading will be impossible..

    the only thing you haven't changed is the stator, i think you've changed ignition coil? plug wire, plug, and plug end? right?

    if you put a fresh plug in does it work ok for a few seconds/minutes then revert back to shitty?

    is it just loading up? can you turn the fuel off and it works again? silencer soaked? does it smoke abnormally?
    #85
  6. Inane Cathode

    Inane Cathode Cheated Anion

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    Plug was dark brown/black this time, new when i left. Was a tiny bit rich down here, brought it up to 10k feet, obviously ran rich then.

    Fresh plug stays reasonably fresh through the whole ordeal, doesnt seem to be loading up. takes on color after 5 miles or so, doesnt look bad or soaked or black. Fuel off it runs great for just a second then dies, im thinking it's leaning way out and its easier to fire for those couple seconds.

    Swapped coils with another bike, runs the same, swapped plug cap, runs the same, wire is built into the coil so that swapped along with the other coil. Only thing i haven't swapped is the stator.

    If it still runs shitty with points i'm not sure what i'll do at that point, really. That would then be literally every system swapped out with no change.
    #86
  7. stainlesscycle

    stainlesscycle Long timer

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    still bunches more things it could be...- bad crank seal(s), bad port hack job, piston in backwards(!) etc etc....
    #87
  8. Twin-shocker

    Twin-shocker Long timer

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    As I suggested some while back, have you tried a carb which is either known to be working good, or is new? But seems to me that bearing in mind the horn issue, that the problem is more likely related to the electrics.

    If it was my bike I would convert it back to inductive ignition, and fit a new OKO carb. I would guess an inductive system for a DT would be very easy to find in the US and new OKO carb is $50 from Ebay, so bearing in mind all the hassle you seem to be having, that might be worth looking at?
    #88
  9. stainlesscycle

    stainlesscycle Long timer

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    reading is fundamental.
    #89
  10. Twin-shocker

    Twin-shocker Long timer

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    Thought the OP had found a used carb from a bike breaker? Would be very difficult to know if a used carb was working properly or not, and a fair number of carbs are going to be blocked with Efuel residues.
    #90
  11. Inane Cathode

    Inane Cathode Cheated Anion

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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.
    #91
  12. JeffS77

    JeffS77 cheap bastard

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    Well at least you said it first so nobody can say they told you so :D
    #92
  13. dhallilama

    dhallilama Been here awhile

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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...
    #93
  14. Twin-shocker

    Twin-shocker Long timer

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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.
    #94
  15. Inane Cathode

    Inane Cathode Cheated Anion

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    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?
    #95
  16. Twin-shocker

    Twin-shocker Long timer

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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.
    #96
  17. Scootern29

    Scootern29 Been here awhile

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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.
    #97
  18. stainlesscycle

    stainlesscycle Long timer

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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. "
    #98
  19. Scootern29

    Scootern29 Been here awhile

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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.
    #99
  20. dhallilama

    dhallilama Been here awhile

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    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...