GS125 ESF ('89) revs hanging

[WeezyA]

Hi guys,
I'm new to biking having just passed my CBT in september, and I bought myself a GS125 which had been stored in a garage for 10 years with very little care and attention. I bought it because I wanted to understand the mechanics of the bike and learn to repair it, so it was a bit of a winter project I guess. I have done a lot of work on it since I bought it (new chain/sprocket set, new brake lines, new brake lever and master cylinder, new tyres, oiled and set the clutch and rev cables to factory specs wrt free play, changed spark plug, inspected caliper for disc brake and replaced pads, and a new battery to boot).

The final issue before I send her off for MOT is that the revs seem to be hanging high. The previous owner sonically cleaned the carb just before I bought it, and the bike will start up and idle quite happily, however, when i rev it the revs do not come back down and i usually hit the kill switch to stop the engine. What could be causing this?

A few ideas I had:

-stuck throttle cable (it doesn't seem to be stuck to me)
-leaky carb rubbers/air leak somewhere
-carb set way too lean

Any ideas how to go about diagnosing and treating this issue?
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GS125- '89

[Fizzer Thou]

With the engine ticking over at the normal setting,spray some WD40 around the carb mounting and inlet manifold.If the revs rise noticeably then you have an air leak.

If you have to have the carb off,check to make sure that the carb slide is around the right way.While the carb is off,use a vernier caliper to measure the float height
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Just talk bikes.What else is there?

Always have a 'Plan B'

[Luckyfish13]

Yep all of the above...lube the throttle and choke cables and with the air filter off do a visual check to make sure the slides fine, checking the spring for the throttle slide if it doesn't return to closed when it should.

Also the fuel and air screws are worth checking and adjusting if necessary a manual or quick search online will find the standard settings (if not try 1.5 turns out for each and go from there)

Maybe give the jets a visual check as well, ultrasonic cleaners are good but there not perfect...
Make sure to use good quality flatheads for the jets defo don't want to be rounding them off!
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Daytona 650
XT 125R

[WeezyA]

Cool, thanks for the advice, I will try all of these this week and report back! It smacks to me of the slide not reseating itself correctly come to think of it. I spent all of last night arsing around with the mixture (don't have any carb cleaner to hand to do any of the stuff you guys suggested, should arrive tomorrow) and it made naff all difference.
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GS125- '89

[WeezyA]

Just to let you know, after ruling out the throttle cable being too tight, I decided to do a carb teardown, as my bike had stopped starting at all, and occasionally I would get fuel spilling out of the overflow pipe. In my mind I'd rather know the carb is fine and go from there, so with my Haynes manual at hand I dissassembled and cleaned it.

As soon as I disassembled it I knew the problem, the pilot jet had a deposit partially blocking it, and the float needle's cradle was manky too which probably explained the intermittent leakage. Luckily at work we have an ultrasonic cleaner, so i bunged all the bits that didn't have rubber in them into that with a load of strong detergent. Half an hour later the carb body and jets looked good as new. As soon as I put them on the bike and set it away idling I could tell it was worth it. No more overflowing, no more surging idle, no more hanging at high revs, and best of all I did it all for free!
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GS125- '89

[Rogerborg]

Winner, nice one and good job on sticking with it.
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Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike