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Thread: 1979 DT400 ? build

  1. #1
    Registered User ls400x's Avatar
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    1979 DT400 ? build

    $312 bones down so far

    1979 DT400 mostly there but disassembled engine
    DT125 with aborted 400 conversion, good runner
    New 1.5mm over Wiseco piston and cylinder bored to suit
    New gasket set and factory parts manual
    Spare 400 wheelset, forks and triples
    2 alternative model 400 fuel tanks

    Not sure what to do with it yet. Street tracker, resto or bare bones fix up for rego. Must be ridable to work.


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    Registered User CA18escort's Avatar
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    Hey you need to get on the vinduro band wagon. Will be up your way in a week but no bike due to moving house bull shit.

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    whaddayatalkinabeet brasher's Avatar
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    you know what to do




  4. #4
    Registered User ls400x's Avatar
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    Vinduro does sound kinda cool hmm so many options. I wish the older yellow tank would for over the frame, fuck that colour scheme looks good.









    Last edited by ls400x; 11-12-14 at 09:27 PM.

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    Registered User ls400x's Avatar
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    Engine swapped into the 400 frame, ignition wiring connected and good spark, new chain and sprockets that came with it put on. I would have started it but I'm lacking a throttle cable.

    I'm leaning towards making it a bit of a sleeper, not bothering with cosmetic stuff but perfect mechanicals, maybe a yz400 cylinder/head/reeds and a bigger carb and fab up a pipe.



    Last edited by ls400x; 12-12-14 at 07:05 PM.

  6. #6
    Registered User ls400x's Avatar
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    Had this thing riding yesterday afternoon, pulls pretty hard and smells good running castor. I've attended to a million little jobs with a fair few to go but it's probably not too far of getting a safety cert.

    Going through the bits that came with it, turns out that the spare crank has a new genuine rod kit in it. Win!
    Last edited by ls400x; 15-12-14 at 01:43 PM.

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    Registered User ls400x's Avatar
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    I have to split the cases on swamp donkey now, the steel clutch plates reacted or something with the fibre plates = rust.

    Ex duration = 178 degrees
    Transfer duration 108 degrees (? low)
    Inlet is "power port", essentially piston port with reeds, duration 296 degrees with the standard window piston or 260 degrees with the MX400 wiseco piston.

    Standard CR500 is 180/124 ish Ex/trans with 360 inlet, big bore no powervalve so should be comparable.

    I could probably modify the cylinder and cases by drilling ports for 360 degrees inlet duration but not sure if it will be worth it. Need a 2 stroke sim program badly.
    Last edited by ls400x; 23-12-14 at 10:01 AM.

  8. #8
    Little engine that could. itsnotagsr's Avatar
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  9. #9
    Registered User ls400x's Avatar
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    swamp donkey sleeper

  10. #10
    needs boost... morerevsm3's Avatar
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    have a few old DT's around here, 1 x 80, 1 x 100 and 3 x 175, indestructible old bikes

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    E36 M3 12.92 @ 108.64mph, N/A 3.0L

  11. #11
    Silly old bugger.. curmudgeon's Avatar
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    Quote Originally Posted by ls400x View Post
    I have to split the cases on swamp donkey now, the steel clutch plates reacted or something with the fibre plates = rust.

    Ex duration = 178 degrees
    Transfer duration 108 degrees (? low)
    Inlet is "power port", essentially piston port with reeds, duration 296 degrees with the standard window piston or 260 degrees with the MX400 wiseco piston.

    Standard CR500 is 180/124 ish Ex/trans with 360 inlet, big bore no powervalve so should be comparable.

    I could probably modify the cylinder and cases by drilling ports for 360 degrees inlet duration but not sure if it will be worth it. Need a 2 stroke sim program badly.
    You can get massive power increases from these without spending much money and without making them too peaky. Widen the ex. port to 65 - 70% of bore width, thin ring Wisecos will go a bit more. Raise the ex. port to give about 190 - 195deg duration, curving the top edge to make the rings live. The existing 35deg of blowdown is good, so raise the transfers just enough to maintain this number. Porting transfers is awkward, but be careful not to change the shape and angle of the roof. Also spend some time smoothing out the short side radii of the transfers without laying them over. Remember that the piston windows do more than control intake timing - the rear transfers usually breathe through them as well so check that the new piston doesn't restrict this.

    You'll want a 36 or 38mm carb, Keihin PWKs are the current hotness but a good old Mikuni VM is fine too and much cheaper. But whatever you get buy it brand new. A stuffer that blends the flow into the reed cage as used on modern bikes will help the top end a bit but you can live without it. I'm not a fan of the mickey-mouse multistage reeds - single stage carbon reeds or even the stock stainless petals are fine. Take the time the lap the rubber sealing faces of the reed cage flat on some plate glass or similar with some fine wet or dry. It won't make it go any faster but it'll help it to start easily and idle nicely.

    Don't get carried away with compression ratio - big bore 2 strokes are always looking for an excuse to detonate or stick the piston and you won't lose much power anyway so be conservative - 10.5:1 (full stroke) is plenty. Try to reduce the squish clearance to about 30 thou. Don't be tempted to stuff the bottom end either; high crankcase compression ratios are just a crutch for poor breathing/piping and will cost power.

    The big thing is the pipe - I think of the engine as just an accessory or a driver for the pipe, which does all the real work. The porting just has to be sufficient to support the pipe, and the pipe determines the power characteristics. Some of the online calculators/designers are a bit dodgy so be careful. I like the current version of 2 Stroke Wizard Tuned Pipe, or anything based on Blairs formulas will work.

    You'll find it'll want a bit less total advance, about 18deg would be a good place to start. If it still has points ignition the Hitachi CDI from the IT should fit and will still run the lights.

    None of this will cost a lot, but it'll make it much quicker.

  12. #12
    Registered User ls400x's Avatar
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    Quote Originally Posted by morerevsm3 View Post
    have a few old DT's around here, 1 x 80, 1 x 100 and 3 x 175, indestructible old bikes

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    Cool! just don't open up the engines if the engines if you are OCD like me

    Quote Originally Posted by curmudgeon View Post
    You can get massive power increases from these without spending much money and without making them too peaky. Widen the ex. port to 65 - 70% of bore width, thin ring Wisecos will go a bit more. Raise the ex. port to give about 190 - 195deg duration, curving the top edge to make the rings live. The existing 35deg of blowdown is good, so raise the transfers just enough to maintain this number. Porting transfers is awkward, but be careful not to change the shape and angle of the roof. Also spend some time smoothing out the short side radii of the transfers without laying them over. Remember that the piston windows do more than control intake timing - the rear transfers usually breathe through them as well so check that the new piston doesn't restrict this.

    You'll want a 36 or 38mm carb, Keihin PWKs are the current hotness but a good old Mikuni VM is fine too and much cheaper. But whatever you get buy it brand new. A stuffer that blends the flow into the reed cage as used on modern bikes will help the top end a bit but you can live without it. I'm not a fan of the mickey-mouse multistage reeds - single stage carbon reeds or even the stock stainless petals are fine. Take the time the lap the rubber sealing faces of the reed cage flat on some plate glass or similar with some fine wet or dry. It won't make it go any faster but it'll help it to start easily and idle nicely.

    Don't get carried away with compression ratio - big bore 2 strokes are always looking for an excuse to detonate or stick the piston and you won't lose much power anyway so be conservative - 10.5:1 (full stroke) is plenty. Try to reduce the squish clearance to about 30 thou. Don't be tempted to stuff the bottom end either; high crankcase compression ratios are just a crutch for poor breathing/piping and will cost power.

    The big thing is the pipe - I think of the engine as just an accessory or a driver for the pipe, which does all the real work. The porting just has to be sufficient to support the pipe, and the pipe determines the power characteristics. Some of the online calculators/designers are a bit dodgy so be careful. I like the current version of 2 Stroke Wizard Tuned Pipe, or anything based on Blairs formulas will work.

    You'll find it'll want a bit less total advance, about 18deg would be a good place to start. If it still has points ignition the Hitachi CDI from the IT should fit and will still run the lights.

    None of this will cost a lot, but it'll make it much quicker.
    thank you! I'm just about to start on the top end. Currently the transfers remain partially covered at BDC by about 3-4mm. With the exhaust duration you mentioned I'm wondering if it's worth just raising the cylinder and see what sort of compromise can be achieved? I previously kinda discounted the idea because I thought 180 exhaust duration was already fairly up there per Bell's recommendations. Ex port is currently 58% of bore diameter.

    I'll have to check what you mean about the transfers breathing through the piston windows.

    I have a couple of PJ PWK's to dummy up I'll have a look at.

    I think the uncorrected CR is 8.0:1 at the moment with 86.5 bore and 2mm head gasket with 86.0 ID, but I have 1.0mm copper gaskets there to. I want to make a boss to screw into the plug hole to hold the head in the lathe, is this the typical method?

    I'm still learning TIG but a pipe building is on the hopefully list. it has CDI but I don't know if it's fixed timing above cranking speed or a curve. The CDI units are supposedly fragile and outright life limited due to the capacitor type used. I have a spare but it would be good to go to something like a rex's speed shop one in the future. http://www.rexs-speedshop.com/CDI-Units-1/Yamaha

    I've got a hold of a Clymer performance book that has some recommended port changes for the 360cc which is also 70mm stroke and "I think" 5mm shorter rod but could be comparable.







    A yz400 cylinder/head/reeds would probably run me $500 at least but this thing running one sounds so so good

    Last edited by ls400x; 02-01-15 at 09:43 PM.

  13. #13
    Registered User ls400x's Avatar
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    Here's where I've just finished. Used a '79 left case and a '78 right as they were the least beat up. It got it buttoned up and the crank was obviously way out of true so fixed that. It has all new seals, gaskets and main bearings (Nachi c3) now.
    Last edited by ls400x; 02-01-15 at 08:52 PM.

  14. #14
    Silly old bugger.. curmudgeon's Avatar
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    Quote Originally Posted by ls400x View Post

    thank you! I'm just about to start on the top end. Currently the transfers remain partially covered at BDC by about 3-4mm. With the exhaust duration you mentioned I'm wondering if it's worth just raising the cylinder and see what sort of compromise can be achieved? I previously kinda discounted the idea because I thought 180 exhaust duration was already fairly up there per Bell's recommendations. Ex port is currently 58% of bore diameter.

    I'll have to check what you mean about the transfers breathing through the piston windows.

    I have a couple of PJ PWK's to dummy up I'll have a look at.

    I think the uncorrected CR is 8.0:1 at the moment with 86.5 bore and 2mm head gasket with 86.0 ID, but I have 1.0mm copper gaskets there to. I want to make a boss to screw into the plug hole to hold the head in the lathe, is this the typical method?

    I'm still learning TIG but a pipe building is on the hopefully list. it has CDI but I don't know if it's fixed timing above cranking speed or a curve. The CDI units are supposedly fragile and outright life limited due to the capacitor type used. I have a spare but it would be good to go to something like a rex's speed shop one in the future. http://www.rexs-speedshop.com/CDI-Units-1/Yamaha

    I've got a hold of a Clymer performance book that has some recommended port changes for the 360cc which is also 70mm stroke and "I think" 5mm shorter rod but could be comparable.

    A yz400 cylinder/head/reeds would probably run me $500 at least but this thing running one sounds so so good
    Spacing up the barrel will work, though obviously you'd have to machine the head to suit. Also check that the bottom ring doesn't pop out into the intake at BDC if you go this way. A threaded mandrel into the plug hole is the usual way of chucking the head into the lathe.

    Remember that the relationship between cylinder size and port timing is similar to that of cam timing with a 4 stroke. In other words, as the cylinder size increases you have to use more port duration for a given rpm range, just as a bigger 4 stroke cylinder wants more camshaft than a smaller one. So while 190-195 ex duration sounds like a lot it's about where you'd want it for a cylinder of this size to make good power. I'm not a fan of Bell.

    The flow path from the crankcase into the rear transfer is via the windows in the rear piston skirt. You have to be careful with non-standard pistons that this flow path isn't disrupted through BDC when the transfer is open.

    Most of the older Hitachi CDIs were fixed curve, and yes the capacitors do weaken with age. Believe it or not one of the best replacement units is a cheap (about $15) Chinese job that gives about 1 deg retard per 1000rpms. If you could get hold of a YZ top end or complete motor for a good price it'd save you some work and you could possibly adapt the YZ pipe too.

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    You 'gon take that deeuck fat910's Avatar
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    They say i know fuck nothing, but i know fuck all!

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    Registered User ls400x's Avatar
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    strong post

  17. #17
    Registered User ls400x's Avatar
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    wrong gasket, had to make one



    fitted like a bought one



    oil pump drive blanking plate because I don't have the original dual throttle cable setup





    I'm using a standard cylinder and piston to get a baseline and because I want to ride it. My 1.5mm over cylinder and mx400 wiseco is a bit tight on clearance at 2 thou and needs a touch.

    transfers 3.0mm below piston at BDC.



    Last edited by ls400x; 04-01-15 at 07:57 AM.

  18. #18
    Registered User ls400x's Avatar
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    riding this big bitch around today







    before I did though I measured the timing for various cylinder heights over standard:

    using dt type 1.0mm oversize piston on the 1978 cylinder

    standard with 0.50mm Yamaha base gasket - ex 174, tr 110, in 304
    plus 2.8mm - ex 183, tr 117, in 282
    plus 3.2mm - ex 184, tr 119, in 281
    plus 3.6mm - ex 185, tr 121, in 279
    plus 4.0mm - ex 186, tr 122, in 277

    the head has about 3.5mm before the fin and 1.0mm copper head gasket. Thinking of sacrificing a head to try spacing a cylinder.

    Last edited by ls400x; 04-01-15 at 08:08 PM.

  19. #19
    Malakia Industries
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    Thats awesome. Even better because its registerable.

  20. #20
    Silly old bugger.. curmudgeon's Avatar
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    Before you do anything check that you aren't going to have any issues with the bottom ring in the intake port at BDC. If everything is ok cut 3.5 from the head and space the barrel up the same amount. You'll pick up a fair amount of top end at the expense of the bottom, if you're happy with the improvement (and you will be) consider widening the exhaust as well - simply raising it is like adding cam duration without any extra lift. It could also do with a fair bit more intake area, but keep the bottom ring in mind before raising it too much.

    But remember, it won't really come to life until it has a suitable pipe, this will make a huge difference.

    edit: my road bike (see the Something a bit different thread) has a very similar sized cylinder and an almost identical porting layout. It runs 196 exh dur. and still has enough low end that you can ride around town under the powerband.
    Last edited by curmudgeon; 05-01-15 at 05:46 AM.

  21. #21
    Registered User ls400x's Avatar
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    thanks! I appreciate your insight. I can raise the cylinder about 5mm total before the ring touches the inlet port. I would definitely like to optimise the port size and shape once I've ridden it a bit with different port timing. Do you have any books or software you would recommend? Your Bultaco is really interesting, do you have a detailed build thread anywhere? I was originally thinking 40hp with ok manners could be the goal for DT but I'm warming to big port durations for something propper angry So many things to optimise at once though! I'm still learning on the lathe and TIG so everything takes a while.

    I found some standard kx500 port timing pre-powervalve for consideration to.


    Year      transfer   transfer        exhaust    exhaust        total   total
                  open       close            open       close           inlet     ex
                  BBDC       ABDC            BBDC       ABDC          DEG    DEG

    85            62           62                92          92             124    184

    86           59.5        59.5               96          96             119    192

    87           59.5        59.5               86.5       86.5           119    173

    88           60           60                 90          90             120    180

  22. #22
    Silly old bugger.. curmudgeon's Avatar
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    Don't focus too much on (or be distracted by) port timing alone. There's a very important number used in two strokes called mean time/area (and the related number time/angle/area) that describes the amount of port opening available at a given speed. It's roughly equivalent to a four-strokes "area under the lift curve" in that lots of curtain area (or multiple valves) can allow much less cam timing for the same rpm range. So more modern engines with multiple exhaust ports (like the KX) or very wide, bridged ports can use a lot less timing than our old engines with a single, oval exhaust port. Timing figures alone - without knowing port area - are meaningless.

    Again, like cam timing, increasing the duration alone without increasing area tends to affect power range negatively without giving a corresponding increase in output. The first step therefore, should always be to increase the port area as much as possible (mostly by widening) to get as much power as possible while narrowing the powerband as little as possible. Modern thin rings can easily handle over 70% of bore width, even stockers can handle 65% without problems.

    Two-strokes in general don't respond well to one-thing-at-a-time mods. Everything has to be singing the same tune so to speak, so what you have to do is pick an rpm range and then size everything (ports, pipe, carb and intake) for that range. For your engine 40hp would be trivially easy and you could go for more if you wanted. The cylinder displacement is enough that the bike would be rideable even below the powerband. I'd probably pick 7500rpms as the peak - it'd make enough power at that to be fun without being too highly strung. Plus it's a lot easier to make holes bigger than smaller.

    Gordon Jennings Two-Stroke Tuners Handbook is a classic that is available free online here. It's pretty old now, and some of the info (mainly the transfer t/a numbers) is outdated, but it's still the best introduction to 2 stroke tuning there is. And for old engines like ours you can use his t/a recommendations reliably. Blairs' Basic Design of Two Stroke Engines is also very good but it's nearly 700 pages of heavy going. Still, there are some nuggets in there if you can stomach it. Pretty much every two-stroke book and software package has its roots in Blairs research. Also check out MacDizzy's Two Stroke Technology Exchange, it's a paid subscription (about $25 a year from memory) web forum. There's not much activity there these days but it's worth it just for all the archived info and the fact that you have to pay to get in helps keep the fuckwits out.

    I don't have a build thread anywhere for the Bul, but I have the engine apart at the moment (for some bottom end breathing improvements) so I guess I could take some pics and write some notes.
    Last edited by curmudgeon; 06-01-15 at 06:05 AM.

  23. #23
    Registered User ls400x's Avatar
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    progress


  24. #24
    You 'gon take that deeuck fat910's Avatar
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    Nice! Got the volume close?

  25. #25
    Registered User ls400x's Avatar
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    I hope so, I paid for some pipe software and made it give me a 3 stage pipe for 3.5mm raised cylinder timing values and 7000rpm peak. I'm no sheety but I have a feeling making the cones was the easy part. I haven't TIG welded steel before so 50:50 it will end up scrap.
    Last edited by ls400x; 16-01-15 at 05:52 PM.

  26. #26
    Silly old bugger.. curmudgeon's Avatar
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    Nice. Don't stress about the welding; I'm still oxy welding mine and they come out ok. A flap disc and some flat black will cover up damn near anything. Be prepared to make some small adjustments to the length after testing - I always make them about 20mm or so longer as it's always easier and neater to cut a bit off one end of the belly rather than add a bit. And think about using an "inside stinger" - it makes managing the noise much much easier and doesn't cost any power.

    The white spatter over everything - you must have really liked the way it turned out eh?

  27. #27
    Registered User ls400x's Avatar
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    Thanks for the advice. I'm thinking of going under the frame to make it easier to build although it looks like it wouldn't fit up too bad following the original routing as it's a bit shorter in length. I'll give the inside stinger a go.

  28. #28
    Registered User ls400x's Avatar
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    Curmudgeon: Ex port width measured straight line or circumferential distance?

    I bit over a month of not thinking about this and I've forgotten half of it. There isn't much thickness between the exhaust port and the base nut bores. The edges are going have to be a fairly shallow angle to get much more width.

    I've got a 2002 Honda CR250 pipe lying around that measured up very close to my new pipe dims. Out of the cylinder is a few mm smaller diameter but length near enough to the same and belly section same diameter. Not sure if I should consider trying it.
    Last edited by ls400x; 19-02-15 at 02:18 PM.

  29. #29
    Silly old bugger.. curmudgeon's Avatar
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    Measure straight across the port window - not round the curve. It's not the end of the world if you have to taper the port in a little from the window; remember that you always have the full area of the port passage available, but the window itself is only fully open very briefly at BDC.

    You could give the Honda pipe a try but you'd have to do something to fix the mismatch at the barrel face. So long as the length is the same it'll peak at the same rpms.

  30. #30
    Registered User ls400x's Avatar
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    exactly 2 hours worth of spacer using dremel, drill, band saw and linisher.

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