Suzuki GN125 idle/stalling problems

[dirtybiker]

My girlfriend has just bought a 2005 suzuki gn125, one of the ones that were made under license in China, and it's really not running very well.

It has two problems that seem to depend on where the idle throttle screw is set.

When it's set too low it will idle at 1,500rpm and go up through the gears just fine, but when coming to a stop, you pull the clutch in and let the revs drop to idle, they just keep dropping until it stalls.
When the throttle screw is set any higher than this, it doesn't stall, but the tickover keeps rising up to 4000/5000rpm when the clutch is pulled in. The throttle definitely isn't sticking open and when the revs aren't sticking they seem to take a long time to drop back to tickover.

Both of these things happen when it's cold and when it's warmed up.

It's had a brand new carb, because we thought that the diaphragm was sticking, but this hasn't helped. It's also had fresh fuel in since we got it(it had sat around a while before) and the air filter has been cleaned and oiled.

Anyone know what's wrong with it?

[Val]

Have you checked for air leaks?

Put the bike in first gear and pull the clutch in and try to push the bike forward you should be able to push it easy. If not you have clutch drag or adjustment issue.

Have you adjusted the clutch cable?
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Adrian Monk: Unless I'm wrong, which, you know, I'm not...
Yamaha Fazer FZS 600, MT09, XSR 900

[RhynoCZ]

To check for air leaks of the rubber manifold that holds the carb/s in the head. Start the engine, make it run at idle, spray something flammable over the manifold. If the revs raise, then it's time for a new manifold. Some cracks can be fixed (glued) but I personally wouldn't do that.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[Val]

[RhynoCZ]

Yep, basically any aerosol spray with flammable propellant would work. When I say any, I literally mean ANY.

Use the starter fluid, will work the best and won't leave any grease or smell on the engine.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[dirtybiker]

The inlet manifold looks a bit worse for wear so I've fixed some cracks with glue(don't think they went all the way through anyway), but I'm ordering a new one just in case.

I'm also going to have a play around with the air/fuel mixture screw incase it's running lean after cleaning the air filter.

I don't think there is an air leak because it runs perfect apart from when it's idle.

I'll try the starter fluid thing and update this when I've fitted the new inlet manifold.

[RhynoCZ]

Well, that's the thing, the cracks in the manifold let air in. If you put a brand new one there, there will be no air leak.

Air leaks are noticeable especially at the idle.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor