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GN125 idle problem


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First I should say that I'm very inexperienced when it comes to fixing bikes (as will probably become apparent later in my post!), but very keen to learn.

My bike is Chinese built, 2006, 18,000 km.

I recently spent a bit of time on the bike. Changed the oil and filter, lubricated and adjusted the chain, cables etc. The choke lever was very sticky so wd40'd that, it turns smoothly now.

Before I lubricated the choke lever the bike would often idle at high rpm (2500-3500) when warm. Now it idles very slowly and often cuts out without throttle. I suspect the choke may have been always on (just a little), a previous owner might have adjusted the idle speed down to compensate for this. Now the choke is off, the idle speed isn't enough...

So it seems I have the simple task of increasing the idle speed. Simple for many, but not for me! A friend suggested I find the idle speed screw and simply turn it. There is a screw/ bolt on the left hand side of the carb that appears to go through to where the throttle cable ends. Not a great description this, but it's the best I can do! The screw/bolt has a spring around it. Anyway, it's impossible to turn this screw. When I try the whole carb moves and rather than break anything I gave up.

I've managed a workaround by tightening the throttle cable at the handlebar end. By taking the small amount of slack out of the cable the bike now idles at 1300. However it's not ideal, revs take a long time to drop, I'm pretty sure this isn't the right way to do this.

Sorry about the very poor description of this, can anyone help?


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Through what you have described, you have found the idle screw. It may have just seized. Give it a good squirt of wd40 and see if that loosens it. Carbs are pretty tough so don't worry too much, just see if you can clamp it steady whilst you turn the screw. Try it both ways to loosen it and just have the engine running (you may need someone to keep a bit of throttle on at first to stop it cutting out).

Then I would loosen the throttle cable again as it's quite important to have a little free play in it.

If you're unsure of anything, take some pictures and post them on here.

Good luck,


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Oh dear. These things happen. How much left of the screw do you have left sticking out? If it's quite a bit, you can file a slot into the end and get a screwdriver blade in there or file the sides and use a spanner.

If it's broken flush or sunken, you can drill it and use a screw extractor.

Are you able to take a photo of the screw and the carb? It would make it a lot easier.

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Is this the screw you broke?



No it's not that one. It's a bolt that runs upwards and to the right at 45 degrees. It pushes on a metal thingy that's also operated by the throttle. So it seems to be a simple affair that holds the throttle slightly on, in order to increase idle speed. Hope that makes sense!

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that definitely sounds like the idle screw, if there is not enough of the screw head to grab hold of you should be able to get it from the back as i'm assuming its just screwed through a thin bracket like the one in the picture I've posted?

if so spray some penetrating fluid on it and leave it long enough to work its way through then go at it with a pair of vice-grips or pliers.

should work its way loose eventually, you then should be able to replace it with a screw of equal diameter.

good luck!


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It seems I had the right screw then. It's different from that picture though, in that the screw goes through the carb itself (probably 8mm of metal) rather than just a thin bracket. I've got some mole grips, I'll have a go with them when I've wd40'd it a few times. There's about 20mm of the bolt to get a grip on, it snapped off at the head. Anything else I can try if it remains stubborn?

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