[Engine stuttering at WOT on a DT 125 MX]

Hi, thanks for the swift responses.

I am away for the weekend but will upload some photos next week of spark plug colours and other details etc next week.

last time I looked the spark plug was a brown colour so thought that was fine. Have changed air filter few months ago too. Im not sure whether the carb is the stock carb for the engine, also not sure if the jetting is definitely correct - it’s a mikuni 4J300? 
 

I lowered the needle jet by two notches, it didnt seem to make much difference.

it also is difficult to start - if its running rich is that the plugs being fouled? 
 

 

 

[Engine stuttering at WOT on a DT 125 MX]

On my DT 125 MX (1981), whenever I go WOT and a reach higher revs in each gear the engine starts stuttering rapidly and won't rev/accelerate any higher. This is annoying because it happens just as I get into the powerband. If I change the spark plug, the bike runs pretty sweet and can rev much higher but then after a few rides it goes back to the stuttering problem. Any suggestions to fix this? Cheers

[DT 125 MX carb slide sits too high...]

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?

[DT 125 MX carb slide sits too high...]

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

[DT 125 MX carb slide sits too high...]

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.

 

[DT 125 MX carb slide sits too high...]

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

 

 

[DT 125 MX carb slide sits too high...]

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