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Discussion Starter · #1 ·
Hi guys,

Newbie here. I find that my KTM SX idles fine on cold start in neutral with choke off, but as it warms up it slowly dies. I have adjusted the idle several times which seems to fix the issue, but next time I take it out same again. I also find that when it warms up it does not idle for long with the clutch in.. it will eventually slow and cut out. I have read conflicting advice online as to whether race bikes should actually idle, but looking for advice. I do not want to keep turning the idle screw to eventually foul the plug. Appreciate any thoughts! Thanks
 

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What sx is it?
 

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Discussion Starter · #5 ·
So back off on the idle screw? Premix is bang on and no issues when running. Choke never needed when starting. Plug not fouled and still starting no bother
 

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If its starting with no choke and running gets worse as it warms up, then its too rich. More to do with jetting than the idle. Try adjusting the airscrew till it runs better. If airscrew needs to be more than 2 turns out, you need a weaker pilot jet. Running rich could also be an indication that the reeds are in need of attention.
 

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Discussion Starter · #7 ·
I’ve not actually touched the air screw but it seems to be right in as far as I can see. I’ll turn it out two turns and see how I go? Thanks appreciated
 

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Go for the typical 1.5 turns out from fully in - go carefull bottoming the screw out. Beyond 2 turns out typically indicates needing a weaker pilot. Anything less than 1 turn out suggests it needs a richer pilot. Get that part of the running right and see how it is ...... if you are otherwise happy, then it is likely that needle and main are fine. Just change one thing at a time so you know what the changes are doing in isolation.
 

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P.S - re. Idle with clutch in and cutting out; assume that that is when its in gear? You might just have a bit of clutch drag. I dont know the 125, but carefully use the level check bolt to get oil level right, then when you change oil, measure what comes out and puf that amount back in, that should reduce the drag. Just get a plastic measuring jug from the supermarket. With the level bolt, be very careful with it, they are a weak point for stripping threads. Just remember they only need 8nm to tighten. Once you know how mych oil it takes, never use that bolt again. As an example, my 2t bike says 800ml in the spec, but once oil level set, l drained out about 675ml ...... so that is all that goes in at oil change - no drag.
 

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Discussion Starter · #11 ·
Unfortunately the oil check bolt is seized, but I recently bought the bike used and with no way to check the oil I did top up 100 ml or so. which may be the issue! Planning to do an oil change before I go out again but I am sure the manual states 700 ml motorex which I have. It is when it is in gear.. it will idle for around 15 to 20 seconds before starting to bog slightly and cut out. Maybe an oil change is the next step. Apologies.. still learning!
 

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Discussion Starter · #13 ·
Thanks for the advice! I will change the oil before I go out again.. manual states 700ml but go with 650ml yeah? I was just worried that there may be very little oil as unable to check, maybe should have just drained it from the start.. hence why I thought I should top it up to be safe. how often would you change the oil? It is not being ridden hard at the moment, just fields really. No tracks or tough off road as yet. Thanks again!
 

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Yes, 650ml would be on the cautious side (personally, l would go 600ml, but stick with the 650ml) l change my oil every 15 hrs. Others may go longer, but the oil is pretty filthy at 15hrs. If the 125 doesnt have an hr meter, they can be got off ebay for about £10 - they just take a reading of spark plug wire. Very easy to install. Fingers crossed that it all improves the running. Also, keep the air filter clean and use a decent filter oil (personal choice is the putoline action fluid, not the spray)
 

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Discussion Starter · #15 ·
That was my next question around the filter oil. 😊 I will get some of the putoline filter oil as advised. I have new filters and will change and oil the new filter at the same time as bike oil change. Any tips on filter cleaner solution to clean the used filters? I will also look into the hour meter. Thank you!
 

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I just use old stale petrol to get the oil and grease off (use spray grease on the rim that seals against the boot) - have done for years with no problem of filters breaking up. After petrol, wash in a bucket with warm water and wash up liquid - rinse through from inside out under the tap. Sounds a work up, but if you do a few at once its less hassle and soon done. I store freshly oiled filters in 'celebration' type tubs so l have them to hand.
 

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What needle and jets are in your carb? 125s love fuel and love strong input. They then reward you with everything you need. 125s have won world level competition in the past so there’s nothing wrong with the CC. Set up is crucial though.

If you aren’t working it then there will be a jet spec to suit but the margin for jetting error is less.
No small bore likes dirty gearbox oil, they use ally plates which add material to the oil. Fresh light gear oil will do you fine if not doing technical riding and dumping the clutch at high revs?

Thoroughbred 125s can be fickle beasts….never forget that.
 

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Discussion Starter · #19 ·
Thanks. I’ll try the oil change and adjustments and see how I go. Unsure about the jetting but should be able to gauge afterwards.. again all new but learning thanks to the advice here 😊
 
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