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Thread: 1979 DT400 ? build

  1. #31
    Registered User ls400x's Avatar
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    Curmudgeon: I'd appreciate your thoughts on the inlet port if you don't mind. I will be using a Wiseco piston specified for DT/MX as a standard replacement though it has a crescent shape skirt rather than the windows in the standard DT. The Wiseco piston reduces the inlet port duration and it will be further reduced with the spaced cylinder. Should I attempt to increase the inlet duration back to standard or maximum duration I can fit in there? It is likely that leaving it as-is, with the Wiseco piston and spaced cylinder the inlet duration will be in the low 200 degrees although I haven't measured it yet. I've got approx 8mm between the inlet port floor and the cylinder base, and it appears I have probably got at least 10mm wider before I start running out of cylinder material.

    Would you expect there to be any benefit in removing the boost port bridge between the inlet port similar to the scans in my post #12? This will not affect the ring ends.

    My piston to cylinder clearance is 0.06mm room temp.

    The reed area grinding is not mine...






    Last edited by ls400x; 18-03-15 at 01:23 PM.

  2. #32
    Silly old bugger.. curmudgeon's Avatar
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    I'd be trying to get the intake duration at least back to where it was; you could split the difference and take 2mm from the bottom of the piston skirt and lower the bottom of the port 2mm as well. Make sure you radius the edges.

    I wouldn't worry about notching out that boost port but I would be having a close look at the exit angle, especially the top edge of the port in the liner. From the photo it looks like the passage angles up until it hits the liner where it levels somewhat. The steep upwards angle needs to continue smoothly right to the port opening. Raising the top edge will be unavoidable but try not to make it too much higher than the other transfers.

  3. #33
    Registered User ls400x's Avatar
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    little bit more progress. I dummied up the new cylinder and piston and spacer (+3.0mm) to double check port timing and how much I need to cut off the cylinder deck.

    189 degrees exhaust, 125 degrees transfer, 118 degrees inlet.
    Last edited by ls400x; 10-05-15 at 01:20 AM.

  4. #34
    Registered User ls400x's Avatar
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    At about 69% ex port width x bore. Lowered inlet port 6mm, 3mm to come off the piston.

  5. #35
    Registered User ls400x's Avatar
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  6. #36
    Registered User ls400x's Avatar
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    A touch under 1.0mm squish clearance diverging at about 0.5 degrees, 50% squish area, 10.65 ish to 1 compression ratio.

  7. #37
    Registered User ls400x's Avatar
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    new vm38 Mikuni and DG yz400 pipe


  8. #38
    Registered User ls400x's Avatar
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    So I've done a heap on this lately. The frame was cracked completely off below the front left engine mount and had another crack in 1 head tube gusset. The crack had previously been repaired by welding some baked bean patches can or some shit all over the place. I carefully ground off the patch and ground out the cracks and welded them up. I extended the engine mount past the nasty stress raiser and tapered it down along the neutral axis. The chain had also worn away a big part of one of the swing arm struts, welded up that to. One of the rear engine mounts had been busted off so I made a new one. The stand had the foot rusted off and the spring anchor on the frame missing so made new parts and welded them in. Painted the lot after blasting back to bare metal.

    I pulled down the forks and inspected then put new seals in. New clutch and brake cables and levers. New engine bolts and shimmed the mounting points to perfect. EBC +15% clamp force clutch springs got slipped in. New seals on the swing arm end of the rear shock and also seals in the brake pedal. New pivot and buffer for the chain tensioner mech.

    I dummied up the pipe and it looks like it will need very little work to get perfect. Just a few mods to the rear inner guard and relocating the battery left to do there. I threw a second hand new ish front tyre on it to.

    I re cut the head to get 0.88mm squish measured with solder to.







    Last edited by ls400x; 09-05-15 at 06:54 PM.

  9. #39
    Registered User ls400x's Avatar
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    Still waiting on the muffler and throttle to arrive but pretty much ready to ride




  10. #40
    Little engine that could. itsnotagsr's Avatar
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    Nice work!
    "If you can make black marks on a straight from the time you turn out of a corner until the braking point of the next turn, then you have enough horsepower." - Mark Donahue Penske Porsche 917

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    "You can never have enough power. I remember when we had Group B cars... THEN we had enough power!"
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  11. #41
    You 'gon take that deeuck fat910's Avatar
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    Where'd the custom pipe go? I was keen to see how it went. Also, i love the VM carbs. I have them and they seem easy to tune even with a muppet like me playing with them.
    They say i know fuck nothing, but i know fuck all!

  12. #42
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    Great work.
    How did you settle on .88mm squish - I thought 400cc would need something like 1.3mm or so squish? (forgive me if you mentioned this earlier- been a while since I read the early posts)

  13. #43
    Silly old bugger.. curmudgeon's Avatar
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    Looking good. A few things to keep in mind as you're tuning it - it'll want a bit less advance than before, around 18 deg will be a good start. You won't need or want a ridiculously cold plug, it'll only foul. A 7 or 8 will be fine. While you're tuning listen for detonation with every bone in your body; these big air cooled engines will seize at the first opportunity. Use 98 fuel. Also remember that these things can det even while cruising on small throttle openings. Make sure the engine is good and hot when you jet too, often an engine that runs det-free at normal temps will rattle and seize as it gets hot. Be prepared to spend a fair bit of time on the jetting and timing; I think it'll surprise you with how quick it is.

    Did I mention don't let the cunt detonate?

  14. #44
    Registered User ls400x's Avatar
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    Quote Originally Posted by fat910 View Post
    Where'd the custom pipe go? I was keen to see how it went. Also, i love the VM carbs. I have them and they seem easy to tune even with a muppet like me playing with them.
    The home made pipe is still happening. I usually get carried away and never finish stuff so the DG pipe was a compromise to get there quicker.

    Quote Originally Posted by fnlow View Post
    Great work.
    How did you settle on .88mm squish - I thought 400cc would need something like 1.3mm or so squish? (forgive me if you mentioned this earlier- been a while since I read the early posts)
    Well I'd like to say experience but this is the first one I have done. Just turned out that way after taking another cut. It's probably up around 11:1 compression ratio now so I really hope it's not too close/too high.

    Quote Originally Posted by curmudgeon View Post
    Looking good. A few things to keep in mind as you're tuning it - it'll want a bit less advance than before, around 18 deg will be a good start. You won't need or want a ridiculously cold plug, it'll only foul. A 7 or 8 will be fine. While you're tuning listen for detonation with every bone in your body; these big air cooled engines will seize at the first opportunity. Use 98 fuel. Also remember that these things can det even while cruising on small throttle openings. Make sure the engine is good and hot when you jet too, often an engine that runs det-free at normal temps will rattle and seize as it gets hot. Be prepared to spend a fair bit of time on the jetting and timing; I think it'll surprise you with how quick it is.

    Did I mention don't let the cunt detonate?
    Hmm this sounds a bit daunting, I've never started from scratch with a new carb/combo.

    I forgot to update I lowered the exhaust port bottom edge to flush with the piston at BDC and flattened it out before I bolted it together. I also tapped and installed a stainless button head cap screw from the outside to block the decomp hole. It is a LOT harder to bump over compression on the kick starter than any of the 250cc motocross bikes I've owned.

    Last edited by ls400x; 19-05-15 at 05:29 PM.

  15. #45
    Silly old bugger.. curmudgeon's Avatar
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    Quote Originally Posted by fnlow View Post
    Great work.
    How did you settle on .88mm squish - I thought 400cc would need something like 1.3mm or so squish? (forgive me if you mentioned this earlier- been a while since I read the early posts)
    There are a million theories on the ideal squish velocity, most of which totally ignore squish area. People whose judgement I trust - in this case Jan Thiels and Fritz Overmar, developers of the 55hp+ Aprilia 125 GP engine - have stated that 1% of stroke is a good place to start. So that's what I use and in my experience it works well. I've found best results from making the clearance as tight as is physically possible and reducing the squish area as the rpm range rises. At high rpms 30% area is plenty; for lower rpms 50-55% works well.

  16. #46
    Silly old bugger.. curmudgeon's Avatar
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    Quote Originally Posted by ls400x View Post
    ..It is a LOT harder to bump over compression on the kick starter than any of the 250cc motocross bikes I've owned.
    Grab one of these from your local Husqvarna chainsaw dealer. They make starting a 400 as easy as a 125.


  17. #47
    Registered User ls400x's Avatar
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    So this thing runs now and has done a few km up and down the road. It fired first kick a started second kick. I need to go down a few sizes on the pilot for starters but it goes OK. I retarded the ignition timing about 3 degrees for now, 25:1 JASO FC oil and BP98. It seems to kick over better than I was thinking so the decomp is unneeded for now (thanks for the tip though curmudgeon).

    The muffler is still somewhere else so it's dumb loud at the moment. It's not slow even untuned so I'm hopeful that it will move properly once I've spent some time on it.
    Last edited by ls400x; 15-06-15 at 09:07 PM.

  18. #48
    Registered User ls400x's Avatar
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    Bit more tuning on this done today. It's still rich everywhere so another order of jets this week needed. It feels fairly quick and I don't think it would have any trouble out dragging my '99 RM250. I hits hard coming on the the pipe, not something that I could get a lazy wheely going in the low to mid and not expect to dump me as it gets happy. It still lugs great and is deceptively fast for the few revs it is doing. It does sign off early though, maybe a combination of rich main and pipe tuned length a fair bit longer than ideal. It feels like it will be fun on the road though, very smooth and fun transitioning onto the pipe.

  19. #49
    You 'gon take that deeuck fat910's Avatar
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    Quote Originally Posted by ls400x View Post
    I retarded the ignition timing about 3 degrees for now, 25:1 JASO FC oil and BP98.
    What is this oil and why are you running it so fat?
    They say i know fuck nothing, but i know fuck all!

  20. #50
    Registered User ls400x's Avatar
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    Caltex Havoline Super 2T, mostly because it's cheap and there is no reason not to. Better heat transfer, better ring sealing etc.

  21. #51
    Silly old bugger.. curmudgeon's Avatar
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    Quote Originally Posted by ls400x View Post
    Caltex Havoline Super 2T, mostly because it's cheap and there is no reason not to. Better heat transfer, better ring sealing etc.
    It's great to see someone resist the current fashion for stupidly weak oil ratios. I think these are a result of the OEMS trying to avoid smoke and plug fouling issues, and while you can get away with them with small bore, nikasiled and water cooled engines the big air cooled, iron bored singles respond well to more oil. Besides, with nearly all the current synthetics being prediluted with mineral spirits or similar at various ratios it's really hard to know how much oil the engine is actually getting. Golden Spectro is about the only one I can think of that isn't watered down, the rest seem to be somewhere around 1/3 diluent. I've had a lot of success with Yamalube 2R at 25:1, though my favourite is Shell M at around 32 to 40:1 (it's undiluted). The compression is measurably higher using this stuff and it's remarkably clean for a castor/synthetic blend.

    Keep tuning, it'll rip your arms out when you get it fully sorted.

  22. #52
    Formerly a stupid_cunt fly510's Avatar
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    This is awesome. learnt to ride on a DT250. Winning.
    RB26......................Again

  23. #53
    Registered User ls400x's Avatar
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    Another jetting iteration and got the muffler mounted. I took it for a trail ride too and scared some nature.







    Last edited by ls400x; 26-10-15 at 08:34 PM.

  24. #54
    Registered User ls400x's Avatar
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    I had the cylinder base gasket push out I think from one of the cylinder bolts lack of tension. Anyway, here are a few photos after about 25L of fuel.






    Last edited by ls400x; 16-01-16 at 12:30 PM.

  25. #55
    You 'gon take that deeuck fat910's Avatar
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    Where is the WOT wheelie videos?
    They say i know fuck nothing, but i know fuck all!

  26. #56
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    This brings back memories of my neighbour many years ago.......the barrel on his DT360 was really badly scored but he managed to find a 7th oversize piston for it, rebored it and reassembled and it started first kick. All seemed good but after 30 seconds or so of running it just started revving like crazy on its own, kill switch had no effect nor did pulling the spark plug lead. He realised that turning off the fuel tap and waiting for it to use up all the fuel in the bowl would take too long so he wheeled it up to the side of his house, put the front wheel straight on to the bricks and kicked it into gear with no clutch......bang! Destroyed the gearbox but at least the engine stopped!

    Don't think he ever fixed it. So too much compression ratio with the big overbore?

  27. #57
    Registered User ls400x's Avatar
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    Quote Originally Posted by fat910 View Post
    Where is the WOT wheelie videos?
    Next time I'm home I'll put one together

    Quote Originally Posted by katokid View Post
    This brings back memories of my neighbour many years ago.......the barrel on his DT360 was really badly scored but he managed to find a 7th oversize piston for it, rebored it and reassembled and it started first kick. All seemed good but after 30 seconds or so of running it just started revving like crazy on its own, kill switch had no effect nor did pulling the spark plug lead. He realised that turning off the fuel tap and waiting for it to use up all the fuel in the bowl would take too long so he wheeled it up to the side of his house, put the front wheel straight on to the bricks and kicked it into gear with no clutch......bang! Destroyed the gearbox but at least the engine stopped!

    Don't think he ever fixed it. So too much compression ratio with the big overbore?
    Maybe assembly error, just lack of tension on one cylinder nut, noticed when I was pulling it down.

  28. #58
    Silly old bugger.. curmudgeon's Avatar
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    Dumb question perhaps... but where does the rear transfer get supplied from? Looking at the rear piston skirt and the intake/rear transfer port it looks like the intake is closed at BDC? Is there another passageway from the crankcase to the port that I can't see or should there be windows in the rear of the skirt to feed the rear transfer?

  29. #59
    Registered User ls400x's Avatar
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    Not a dumb question, the rear transfer doesn't get fed from the crank case except for a small duration while its uncovered before the piston skirt covers the inlet port. YZ/MX/IT/DT 400 are all similar in this respect.

  30. #60
    Silly old bugger.. curmudgeon's Avatar
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    That's weird. I remember working on some DTs/MXs back in the 80s and they had windows in the piston. I'm absolutely positively certain that the blue tanked DTs (for one) had windows and it doesn't make sense for them not to have them.

    Edit: Just remembered you'd already mentioned the piston skirt thing. It just doesn't make a lot of sense that the rear transfer would be unfed for a period around BDC; I'd be inclined to cut a hole or holes in the skirt. I'm sure the MX would've originally had a skirt/port height/rod length combo that kept the intake open.
    Last edited by curmudgeon; 18-01-16 at 05:26 AM.

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