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Makotosun

DT 400 B shunting

  • p.wells
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DT 400 B shunting was created by p.wells

Hi all out there, can any of you help in any way , I have fully restored my USA DT 400 B and is now up and running, oil pump is all set up to spec as is the carb with all standard jetting . A  new piston, rings, piston pin, small end bearing all fitted along with both crank seals, I have also fitted a Rexs speedshop electronic ignition system. The bike starts 1st or second kick and idles and runs great until you come off the throttle, then the shunting back and forth starts, its almost like its misfiring. The plug is a nice light tan colour which tells me that the engine is running fine, its just this annoying shunting that spoils the ride. Any suggestions would be a great help guys thanks.
12 Jun 2022 04:53 #1

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Replied by Gr8uncleal on topic DT 400 B shunting

Various threads on this topic and I believe that, whilst irritating, it is usual.
12 Jun 2022 05:04 #2

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Replied by Schu on topic DT 400 B shunting

My DT400B does it too. It’s the only one of all my bikes that does it.
Schu

CT1B, CT1C, JT1, JT2, DT360A, GT80B, DT100B, DT125B,
DT175B, DT175C, DT250B, DT400B, Z50, SCR950

Someday, you'll own some Yamahas
12 Jun 2022 07:22 #3

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Replied by turbodan on topic DT 400 B shunting

Turn your idle speed down as low as it will go without stalling at a stop, that will help reduce the tendency to fire sporadically while coasting down. 

These big singles are difficult to get jetted just right under 1/4 throttle.  You are probably experiencing the usual four stroking at low load.  I would try adjusting the needle clip position.  Raise the clip a notch and see how it runs.  It is not unheard of to end up making a shim to hold the needle a half step between two notches.  It's a fine line between four stroking rich and lean with pinging, you want to be right in between.
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12 Jun 2022 17:13 #4

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Replied by msavitt on topic DT 400 B shunting

my dt360a did this big time.  I found that a 55 vs 50 pilot jet and everything else stock minimized. 

may want to try one size up 
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12 Jun 2022 18:15 #5

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Replied by RT325 on topic DT 400 B shunting

Ya want the throttle hooked to the kill switch so when you shut the throttle it kills the spark till under 1000rpm. Ok maybe not--shut up me.
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12 Jun 2022 19:02 #6

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Replied by msavitt on topic DT 400 B shunting

RT - like an antique hit-miss engine!
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13 Jun 2022 09:53 #7

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Replied by p.wells on topic DT 400 B shunting

Hi msavitt thanks for your suggestion I will get a next size up pilot jet and give it a go, Turbodan also suggested that I go up a notch on the needle clip, in the meantime I have changed the mixture screw from 1 1/2 turns out to 1 turn out and it has improved a bit, not perfect but better than it was.
13 Jun 2022 12:14 #8

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Replied by p.wells on topic DT 400 B shunting

Hi TURBODAN ,  thanks for your suggestion I will experiment with the clip and see how it runs, am I correct in thinking  that if the clip is raised it will make the mixture richer , if so would I need to return the mixture screw back to 1 1/2  turns out.
13 Jun 2022 12:27 #9

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Replied by Sneezles61 on topic DT 400 B shunting

I watched a vid and I believe Mikuni oz from down under… The gentleman gives you a series of adjustments to do and in an order.. I will so be using that as my template to get my RT running better…
Sneezles61
13 Jun 2022 13:55 #10

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