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maximokmo

DT 125 MX carb slide sits too high...

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Posted (edited)

Hi all, this is my first time on the forum so thanks for any answers in advance.

I have recently bought a DT 125 MX 1980. The idle screw has no effect on the idle of the bike (it idles so high it is basically screaming). I took apart the carb and noticed that the carb slide sits really high, maybe a third to half the way up the venturi chamber. Obviously this is the reason for the ridiculous idle. The bike has custom handlebars and forks from a more modern DT. Maybe when they were changed, the throttle cable was too small? How can I lengthen the throttle cable so that the carb slide can sit all the way down? The hex nut adjusters are all fully tightened so I can't change that. I tried cutting about half an inch of the housing of the cable off but the inside of the housing is protected by a metal coil which I can't cut through... might get the angle grinder out. I don't want to change the whole cable itself because the throttle cable splits into two, one to the carb and one to the oil injection system and it would be too complex to change imo. Also the carb slide is the correct orientation and direction (groove lines up with tiny nipple in carb) so it isn't that.

Any suggestions of what to do?

Sorry for the long paragraph!

Cheers, Max

 

Edited by maximokmo

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Is the cable fitted correctly at the twist grip, its possible that the cable is in the wrong notch. Some have a push pull set up and its common for yam to use the same parts across the company. I think i would check the twistgrip, might be worth a lookie see in the connection midway where the oil pump connects with the throttle. Some self adjust, could also be jamming.

Dont try and shorten the outer, you will never cut it clean enough to not damage the internal cable.

Your right fitting the carb slide the wrong way round is a very popular cock up.

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Cheers for the swift response.

I checked the twist grip and it's definitely not that. 

I then checked the midway throttle splitter as you said and had a look.

It isn't jammed but seems like there is a way to adjust the circular bit where the two cables (one to carb, one to oil pump) going out connect into. The throttle cable takes the longer route around the circumference of the circular piece, maybe I could swap their positions? I don't want to affect the oil pump though.

Any idea of whether these things are adjustable and how you do it?

The images below show what I'm talking about plus a photo of the carb slide sitting too high when throttle closed.

https://imgur.com/a/khpdrji

 

 

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hi there,  Ive just had a look in the past forum and found this ...

might be useful?

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Cheers that is useful. My junction box is full of cream coloured greasy gunk and that past thread mentions only using wd40... Might clean it out.

If anyone has any more knowledge on this box, any help would be appreciated. Can't seem to figure out properly how they the circular housing works to be honest.

 

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If you disconnect the cable " does the slide drop right down, ?   ,just to  isolate the cable,,  i have found aftermarket  cable are crap,   the white  cylindrical  piece  ,[ where 2 cables slip into, the slots aint deep enuff , i modified   one with hot knife , to allow cable to slip lower ,, 

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The slide drops all the way down when there's no cable. If your talking about the white circular piece in the junction - the wires slot in fine from looking at it, it's all stock cables I'm pretty sure. 

One other question: when I blip the throttle in neutral, the revs take a few seconds to return to idle revs, rather than a nice quick rev sound. I this common with bikes that are aged? Not much of an issue just doesn't sound great haha.

Cheers for all the help

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Revs  hanging "  can be an air leak  .[ r/h  crank seal]   try closing the air mixture screw , half a turn  ,see what happens,

If your  cable is stock, then back off all the adjustment, [ at bars and  carb top ,]

 

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"junction box is full of cream coloured greasy gunk"

They all are.! it's a type of lithum grease which is white and specially designed for use where plastic is used. Oil based products can cause plastics to degrade when in constant contact with it, they tend to go brittle. WD40 may well clean it but it was packed with the white grease to maintain lubrication over time, something which WD40 won't do as its just a relatively short acting lubricating spray.

Lithium grease is readily available.

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On 6/30/2020 at 3:33 PM, maximokmo said:

The bike has custom handlebars and forks from a more modern DT. Maybe when they were changed, the throttle cable was too short? How can I lengthen the throttle cable so that the carb slide can sit all the way down?

I've just re read your first post. When you say small, i think you mean short? yes?

I think herein is your problem!  If the controls now sit higher up than the original bars then presumably this is putting the cables under tension and raising the slide?.

If it's under tension the outer cable will feel taught with no slack. Is that what is happening?  Remember also when the front end bounces on the fork travel,  the cable needs to be 'long enough' to accommodate this.

(Unless you can find a longer dual throttle cable from a taller bike which is similar to the 125, the 'only' answer to rectify this may be (in my opinion) is to revert back to stock handlebars and forks.?)

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Blackhat250 I will try fiddle around with the air screw and see if I can fix that.

 

NE0 that's interesting about the grease, I guess I will be keeping it in there then. The cable has a fair bit of slack, i.e it isn't under tension so I'm not sure it's that. i'll send some photos later to show you to make sure. I won't be able to revert back to normal as this is how I bought it and it would be a pain to find the pieces and do the change.

One last thing I've noticed with the bike is that at hig RPMs, the bike starts stuttering and therefore you can't rev all the way through the powerband. It will make a stuttering sound and constantly jerk back and forth. Is this an ignition or fuel flow issue or something?

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The  Stuttering,  .  drop the needle ,, [  ie  clip up a groove drops needle, ]

 

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