Suzi GN125 fuelling/idle problems

[ajc219]

Hello, first post here. Just doing my DAS, passed Mod 1 and now running around on a 125 to keep my hand in 'til a second attempt at Mod 2 in July - failed last week for a distinct lack of lifesavers

The 125 in question is an 11 year old Suzuki GN125, 500 miles on the clock and £100. It has spent most of it's life sitting in a shed, so is in pretty good nick (passed MoT no issues) apart from the tank and carb as it was put-away with petrol in the tank...

I've de-rusted and Jenolited the tank, new fuel filter and all is fine there. The carb, a Mikuni CV, was pretty gunked-up so I stripped it and cleaned everything in an ultrasonic bath and then with compressed air. Carb is rebuilt and back on the engine but whilst it runs, it's not happy:

Starting can be a bit of a job, sometimes choke needed, sometimes not. Once started it starts it sits at 1,500rpm, no choke quite happily.

The problem is more when I'm riding, I'll come to a stop at a junction, clutch in and the engine's idling at 3,000-4,000rpm. Throttle is closed, I checked that. Sometimes it slowly drops down to 1,500rpm, other times it just sits there until I move off again.

I've checked for air leaks and that's all fine. The pilot mixture screw was originally at 1 turn out from closed, so I reset it to the Haynes spec. 1 and 5/8 turns and then tried to tune it by adjusting for maximum revs. The problem with that is the engine just seems to get faster the more I unscrew the screw - there's no obvious point of max revs. I also noticed that it's now idling at 1,500rpm when I start the engine and the throttle screw isn't even touching the throttle arm, so I guess the butterfly is fully closed.

I'm pretty sure this is something to do with fuelling - ignition looks fine, sparkplug is a nice light tan colour and there's a reasonable spark when I crank her on the starter motor.

At the moment this is effecting my riding as I need to worry about the engine when I should be worrying about the road and getting all those pesky lifesavers in. I'm tempted just to buy a new carb, they're not too pricey, but I hate admitting defeat!

Thanks,

Andrew.

[Odie]

make sure the throttle cable has free play, i.e its not tight and pulling on the butterfly linkage and holding the throttle open slightly.
Also, reset the mixture to the stock setting, the mixture screw only effects the carb at tick-over and up to about 1/4 throttle at a time.

here is some info that might help or it might not.


Pilot fuel mixture screw settings, float level and pilot jet size also affect high-rpm, 0 to 1/8 throttle maneuvers. Too lean, will cause surging problems when the engine is operated at high rpm at small throttle openings! Opening the mixture screws and/or increasing pilot jet size will usually cure the problem.
NOTE: A rich problem gets worse as the engine heats up.
If the throttle is lightly "blipped" at idle, and the rpm drops below the set idle speed, then rises up to the set idle speed, the low speed mixture screws are probably set too rich: try 1/2 turn in, to lean the idle mixture.
NOTE: A lean problem gets better as the engine heats up.
If the throttle is lightly "blipped" at idle, and the rpm "hangs up" before dropping to the set idle speed, and there are no intake leaks and the idle speed is set at less than 1000 rpm, the mixture screws are probably too lean: try 1/2 turn out, to richen mixture. Be sure there are no intake leaks and the idle speed is set at less than 1000 rpm
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You are so ugly when you were born the doctor said "I?m gonna drop it, if it falls is a rat, if it flies is a bat."

[Noxious89123]

Interesting... My otherhalf has a GS125, same engine as the GN. Her bike will also idle at around 4k when it's warmed up. It's currently dead atm, will start and run for a short while, but then dies and refuses to start for a while, lots of afterfiring whilst cranking the engine over. Got a new plug, same problem. Think it's running rich, as the old plug was black (and dry) when removed.
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'06 Honda CBR125RW-6 ~ '00 Honda CBR600F-Y ~ '07 Honda CBR600RR-7 ~ Bikeless
'53 Ford Ka 1.3 ~ '03 Vauxhall Astra SRi 1.8 ~ '52 Vauxhall Astra SRi 2.2 ~ '53 Vauxhall Astra GSi

[Odie]

check the diaphram in the top of the carb, i have a EN125 to work on at the moment (same engine as the GS125), the diaphram has more holes in it than the tory party!!!!
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You are so ugly when you were born the doctor said "I?m gonna drop it, if it falls is a rat, if it flies is a bat."

[Paddy Blake]

[ajc219]

Thanks everyone - I'll take a closer look at the diaphragm. It seemed fine when I stripped the carb, but I didn't give it too much attention to be honest.

Cheers,

Andrew.

[cb1rocket]

[Odie]

[ajc219]

On the HS26 it's in to lean, out to richen - the screw adjusts a small fuel jet just downstream of the throttle butterfly.

Anyhow, I've pulled the carb apart again and the diaphragm looks fine - it's still soft and I can't see any pinholes when I hold it up to the light. I've also checked the float height and it's bang on the spec, 21.4mm. Blowing through all the passages seems OK, so I really am at a bit of a loss. The only thing that seems slightly sticky is the float needle valve which doesn't always drop back through gravity when I lift and release the float. I'm pretty sure it would be fine when it's got the head of the petrol against it, but I'll check tomorrow holding the open carb over a jam jar and see if it flows fuel.

By the way I found very in-depth service instructions for the GN125 in what looks like a scan from a French bike magazine. It's all in French which is a bit of a pain but the pictures are good. It's a bit of a sod to download as it's on Rapidshare but if anyone wants it, I can stick it on-line for downloading, just let me know.

Cheers,

Andrew.

[Noxious89123]

Can you post / PM a RapidShare link? I have an account for that, so not a problem.
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'06 Honda CBR125RW-6 ~ '00 Honda CBR600F-Y ~ '07 Honda CBR600RR-7 ~ Bikeless
'53 Ford Ka 1.3 ~ '03 Vauxhall Astra SRi 1.8 ~ '52 Vauxhall Astra SRi 2.2 ~ '53 Vauxhall Astra GSi

[ajc219]