Some DT400 Jetting Stuff

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

  1. Tim McKittrick

    Tim McKittrick Long timer

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    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.
  2. klp

    klp Been here awhile

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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...

    :norton
  3. Inane Cathode

    Inane Cathode Cheated Anion

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    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 :lol3

    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 :wink:

    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?
  4. Twin-shocker

    Twin-shocker Long timer

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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.
  5. Valleyrider

    Valleyrider I Survived The '60s

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    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.
  6. anotherguy

    anotherguy unsympathetic

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    He's high altitude. It has an effect on compression.
  7. LasseNC

    LasseNC XSessive!

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  8. Inane Cathode

    Inane Cathode Cheated Anion

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    It's buried a bit, but i did exactly that. Got a yamaha jog cdi box (ac two stroke cdi) and put it in, and it ran exactly the same.

    It's a little low for this altitude. From experience the highest i've ever seen here is right around 100psi. I know it needs a new top end (piston slaps pretty good at idle) but im waiting to make sure everything else is fixed before i go on a top end adventure. Add up it being a smidge worn out, being a two stroke (meaning half the charge is blown back out the exhaust anyway) and low altitude, and i'll believe a 90psi comp test isn't terrible. Then again maybe it is, who knows :p

    So, update time. Got the points plate and rotor last night, was expecting it mondays but it shows up early, neat. Put it in this morning, got the points gap right (i have to love, btw, yamaha's timing advice on the subject "if the stator has been removed, put it back with the screws centered"). Got everything ghetto rigged back together and took a rip down the street.

    I know i've said it before in this thread but it certainly does feel like it's running good. It's pulling hard enough to let me know my clutch cable is too tight and slipping the clutch. Once i had it adjusted right i could whack the throttle open and the front wheel jumps up a few inches, hadn't done that before. Sounds very healthy, and only starts to four stroke out at really high rpm (probably running out the spark advance, i suppose i could screw with that a little bit down the road if i wanted to, low air pressure means you can get away with crap like that up here, lol). It revved a little like it did before under no load (revs to the moon then backs off and sounds like the plug/pipe is loading up) but under load it performs great. Once it was totally warmed up theres no throttle/rpm combination i could put it under that would make it run like before.

    Maybe it's just faking me out but it feels like it's finally fixed. Woot! Now to figure out how to wire this thing ><


    As a funny post script, guess what just rolled into the shop last night? A 1976 yamaha IT400 that "Seems like it has spark when you kick it over but it just starts really hard and runs like shit otherwise" :bluduh
  9. Twin-shocker

    Twin-shocker Long timer

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    I take it that fitting an inductive ignition has cured the problem then?
  10. Inane Cathode

    Inane Cathode Cheated Anion

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    Woohoo! Seems like it's running just fine now :)

    A little bit "flat", doesnt seem to "wake up" but it does put out some torque and it didnt run like dog shit again. Couple pictures from today:

    [​IMG][​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    A few things i'd like to fix: Main feels a little flat, i think it's a bit rich. I'd like to change the gearing quite a bit lower, feels like i have to fly everywhere to be able to get up hills. The kick starter detent ball and spring are gone, that gets annoying. I'd also like to finish up the electrics on it. I went ahead and burned out every bulb on the bike by not having a battery installed, so that's good. I'm considering just going to regulated A/C instead of jacking around with the whole rectifier and battery arrangement..
  11. dhallilama

    dhallilama Long timer

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    awesome that it's a rider, now! congratulations!

    for what it's worth, i use a tympanium 12v ac regulator on damn near anything with a stator and no battery. typanium makes the MSR regulator:
    http://www.motorcycle-superstore.com/2/9/195/7315/ITEM/MSR-Voltage-Regulator.aspx

    cheap, easy to use, reliable.
  12. LasseNC

    LasseNC XSessive!

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    The powerdynamo is an all in one solution there then.
  13. dhallilama

    dhallilama Long timer

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    at US$324 (€243 without vat) + shipping/customs + unavailable replacement parts without getting from overseas... while the unit itself sounds good, it's costlier than some bikes here :)

    wonder if ricky stator or other local-ish company makes a nifty stator for DTs?
  14. Inane Cathode

    Inane Cathode Cheated Anion

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    Well, crap. Hopped on the bike at work to blast over to the parts place to get some stuff. Bike sounded normal, ran ok, all that.

    Got to the store, got the stuff, got out. Started the bike and ran up the street about half a block when suddenly the bike's normal piston slappy noise sounded a little louder and a little weirder followed by KA-CHUNK. Seized motor.

    By the sound of it the piston skirt broke and sent chunks into the bottom end. Awesome.

    That'd make every single part on this bike being broken in some way. Well, actually, the frame hasn't cracked yet, the wheels are generally straight, and the fender's havent fallen off.

    Timing is pretty awful as well, wife fell off a horse and got a pretty nasty hit in the head, hospitalization and no work for a month or so while things de-scramble. Seems like fall has suddenly arrived as well with snow up in the high country. Oh well.
  15. dsm

    dsm n00b

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    Well I just came across this post and I have to tell you a quick story about my 1975 dt 400.

    I fixed it up got a title for it and within 100 miles the piston blew up and bent the connecting rod. Fixed that but the shop bored it out wrong . Got is bored out again ok now. Then one day no spark. turns out the pickup coil overheated for who knows why possibly age. Cannot find a pickup coil. I thought of a way to rewind the coil and did it but due to mounting problems I switched to the DT 250 points ignition (pretty expensive). But now I know a way to mount the pickup but I already have the points ignition soo..... Thing never ran correct just like yours......... Seems to be running lean at mid to open throttle since I hear pinging but it idles pretty good. Well I let it sit outside for 3 years and I just got it back on the road but I have this problem with the pinging and lost power. Found my fuel tank was all messed up with rust. I need to clean that out AGAIN


    I went through A LOT OF BS with this bike and 8 years later I am sstill trying to figure it out LOL! Hey we should get together and bring these bikes and burn em LOL!!!!

    Just kidding I will be in my death bed thinking "how can I get that dt 400 to run like is suppose to ,,,Well I ahve a plan for mine......

    1. Clean fuel tank again ,,,HARDCORE this THIRD time
    2.Clean carb AGAIN (To rid that ethonal left overs I think muratic acid dilutted is the best but you must be careful and kill it with baking soda) I already destroyed a carb with that stuff. The trick is to dilute the acid like 70% water 30% acid and keep checking the progress once all the white stuff is gone use baking soda to kill the acid. Cannot lewt carb sit in the acid long just a few minutes
    3. Go back to stock jetting
    4.Clean reed valves (I guess gum cutter)
    5. Check carb boots for leaks and I mean really check those bastards
    6.I guess I should by a new points and do timing since the one that I had in there got all messed up since I let the bike sit ouside . I had no spark again but I sanded the points and got spark.


    This sucks

    I have replaced all the seals and even made a spring for the decompression valve since you can't get that stupid spring anymore from yamaha . I'm telling ya my dt400 has the same type of issues yours has.

    I had ricky stator recoil the dt 250 coils and they did them wrong (blew my headligh out) so I had to buy NOS ...... Yea lot of money and a big mess with everthing kinda like your hell.

    Now for your bike I wanted to ask you did you check the fuel tank Because just A little bit of rust can cause a lean mixture since the small particals get through the fuel filter and into the carb or just partly clog the fuel fliter. Yes it sounds almost NO WAY but I just witnessed it. Once you throw the reserve on that is when it sucks any debris into the carb even with the filter. Very fine rust particals. I wonder if they can attach themselves to the reed valves and cause a fuel mix problem. I think that is possible. These 2 strokes are very sensitive .

    P.S. Don't use regular oil in the transmission as the book says it's ok. Yea it was ok to use regular oil 20 years ago but now the regular oil has additives that willl destroy the clutch plates. Yea I need a new cluth now
  16. dsm

    dsm n00b

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    Hi! I came across this post again and it looks like it ended sadly but I wanted to update that for me anyway . My 75 dt400 runs great with the points ignition. Has plenty of power. I now have a 78 dt 400 and it runs great with the cdi system but the strange thing is that both bikes, the 75 and the 78 have a pinging issue AFTER THEY ARE FULLY WARMED UP.. (after 10 minutesd of hard offroad riding) I think ethonol in the fuel is causing this. I am thinking of using race fuel or maybe there is a cheaper solution like adding lead subtitute or maybe extra 2t oil to the fuel to keep the random firing of hot spots in the combustion chamber down caused by the ethonol fuel.. What do you guys think? I found a place that reworks the head of theses bikes (and others) so you can run ethonol without pinging issues, like $200.00 to have it done. Race fuel is so expensive!!! I may try the race fuel and see what happens.

    Just think about this problem for a sec. Two vintage 2 stroke enduos run GREAT with no problems until FULLY warmed up and they both act the same way. ALL SYSTEMS HAVE BEEN TESTED AND ARE WORKING LIKE THEY SHOULD. I am thinking ethonol in fuel = bad for 2 stroke air cooled dirtbike.

    Even with the pinging after warm up they still run strong but seem to loose a little punch but not much. I will be back and let you know after I run the race fuel .. Any other ideas from you guys would be great. Thanks
  17. joexr

    joexr Banned

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    What octane are you running now ? Our 93 super here has no ethanol . Yes , 2 stroke oil will increase octane . Try readjusting your pump cable a couple of turns tighter .