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Dt400 surging

13 Jan 2020 00:12
Hind65
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Dt400 surging #1
Hi, can anyone help with my problem, when slowing down at 3500 revs down to 2500 revs the motor surges, after that it settles down to run even. I have rebuilt the motor with genuine piston, new crank seals, I have tried different reed petals and different carb, also tried jetting up and down, still the same! The bike runs great otherwise but this really is testing me! Anyway advice most welcome thanks.

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13 Jan 2020 01:34 13 Jan 2020 01:38 by RT325.
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Dt400 surging #2
If you mean it fires a few times on the way down with throttle closed i remember the 360's being bad for that--the early 360's not so much the DT360A shape which is a for-runner of your 400. But i don't know if we ever truly sorted one. Mate that bought mi near new 360 [RT3] tried leaning off the pilot jet size with some improvement as we decided it was sucking via the compensating pilot hole [which its meant to on partial throttle between idle & cutaway on slowly moving off from a standstill]. So think he leaned that jet quite a bit then had to run the air screw richer. Not a 100% cure. Also if the needle [slide needle] is too rich [in which case it'll be blubbery cruising] then could drop needle a notch [clip up] or check 'that' & the metering part of the jet [emulsion tube] for wear as have seen then wear egg shaped--but talking old MX bikes more than road/trail. Maybe a leaner slide cutaway--like a 3cutaway instead of 2.5 but take all that with a pinch of salt because big motors don't like to be lean or they'll seize up.
That's providing we're taking the same problem in regards to surging?. More i think about it maybe needle down a notch [clip up] & try it seeing its in the needle area & only for 1000rev drop.
Now i've confused it all someone else can have a go. Cheers.
Just thinking--backing the throttle stop off will cure it but won't idle of course. Have mine like that but mostly off road. Can you afford to lower the idle without it stopping.

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13 Jan 2020 03:43
MarkT
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Dt400 surging #3
Very common complaint on the big bores... even magazine tests commented on it going back to the early 360's.

Lots of posts here about this... Search forum for "surging".. Here is a link to a couple of the better threads and has some suggestions others have done to minimize...

yamaha-enduros.com/index.php/forums/74-7...-up-to-snuff?start=0

yamaha-enduros.com/index.php/forums/74-7...c-cruise-surge#85627

(Make sure if you change the slide you don't use a short one with the tall cap unless you make a spacer or convert to the short cap.)

1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

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13 Jan 2020 06:27
Hind65
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Dt400 surging #4
Hi, thanks for that in depth answer, I can afford to lower the idle, even if it cuts out it will be worth it because it's really spoiling a good bike! I'll let you know how I get on.

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13 Jan 2020 08:39
Pedalcrazy
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Dt400 surging #5
Hey partner I feel your pain! I rebuilt a 76 DT400 from the ground up and had the same issue. I posted a lot of good information on the bucking and surging link Mark T. mentioned. I’ll shorten down for you. I had the same issue as you described. Really thought I did something wrong on rebuild. Re-changed everything...timing, carbs, Jets, needles, another spark plug, even bought a nos CDIfor $300 and guess what...no change. I researched old cycle magazines that reviewed the 400 when it first came out and even they commented about “lurching”. So I think we are dealing with a factory flaw! I’ve eliminated about 95 percent of my lurching. My tips are: change the slide from stock 3.0 to 2.5. Got mine at Summit Racing, turn the air screw in as far as you can for the richest idle you can get, turn idle speed down as far as you can get 1300 if you can. These three things helped my bike a bunch. You’ll need to lower the needle (raise the clip) to compensate for the richer slide. Let us know how you come out!

1978 DT400E
1976 DT400C
1973 RT3
1971 RT1B
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15 Jan 2020 12:12
FrankC
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Dt400 surging #6
My 78 400 always bucked up like that, but doesn't after the port work. I think it is common.

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15 Jan 2020 13:56
Hind65
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Dt400 surging #7
Hi guys, thanks for all your advice, I've lowered the idle as far as I can, went for a spin up the road and I can say there is a vast improvement, didn't go too far as it's damn cold here in Lincolnshire! Forgot to mention that I bought a new flat slide carb that came from China, £23 delivered!(sorry if it's not the real thing)! This carb makes it run better than the stock one especially now I've sorted the idle. I struggled to ride it with original carb because of the surging in slow traffic at 30mph.
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15 Jan 2020 14:30 15 Jan 2020 14:44 by FrankC.
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Dt400 surging #8
Were there any difficulties getting the flat slide carb to to work? What's the skinny on it? Can you post a link?
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16 Jan 2020 00:19
Hind65
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Dt400 surging #9
I saw on good old eBay a direct copy of a Mikuni round slide from China which I ordered, guess what, when it came a month later it was a direct replacement for a Keihin flat slide, so rather than send it back, it did only cost £23, I thought I would try it. I had to drill the top threaded adjuster hole out for the throttle cable ferule to fit in but the cable hooked up perfect, changed the pilot for a 50, can't remember what the main was, needle clip in the middle. The back of the carb is a bit larger diameter but the air filter rubber still fitted with a stretch. The motor runs a lot better in slow traffic than stock carb and accelerates far better. Sorry I don't know what you mean by skinny!!
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16 Jan 2020 06:32
MarkT
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Dt400 surging #10

FrankC wrote: My 78 400 always bucked up like that, but doesn't after the port work. I think it is common.


Frank, part of the factory fix on one on the earlier 360 models included minor porting changes... raising the exhaust port slightly if I remember correctly... very slightly... like 1 or 1.5mm.

1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

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