High revs in neutral after throttle release?

[Kernowskinz89]

Afternoon everyone, I recently acquired a 1984 Suzuki GS 125;
A lovely bike and my first geared one after two years on mopeds The bike is original except for the rear shocks, seat and the air filters

Naturally I got taxed and insured right away and got out riding, teaching myself ( I know opinions vary on teaching yourself to ride geared )
I’ve been practicing my clutch control and trying to master the art of downshifting coming up to lights and junctions or when I intend to stop at my destination. Lots of stalling as you can imagine at the moment
But an observation I’ve made is the bike idles fine when in neutral, but if I give it even a small amount of revs it will climb, come down normally but then suddenly climb of it’s on accord and go high.

I suspected the throttle cable and assembly which seems fine and I do suspect the carb too as it is the original Mikuni and has probably never been cleaned or maintained.
However I also have a theory as the previous owner fitted new stock filters before the sale which don’t appear to have been oiled.

Could a surplus amount of air into the carb be causing the imbalance?
Be gentle with me, I’m a novice and taking it on myself to learn all this.
I have a Haynes manual too which has been a bible so far but this is a grey area for me at the moment.

Thank you kindly in advance.
Happy Sunday to all.
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Adrenaline has a colour… and it’s brown.

[Pete.]

The pod filters won't have helped. Whomever fitted them was probably in the misguided belief that it would yield more performance but most likely it didn't, just reduced driveability. If the original has been put back properly, then it should be fine so long as the piston has not suffered scoring from using the pod filter.

First thigs first, check the carb rubber, the one between the cabr and the engine. If it leaks air, that will cause your symptom of holding revs up. Check it's tight both ends and has no splits right through. You DON'T want any air leaks between the carb and the engine. If that's ok check the throttle cable moves perfectly freely. Release the carb top and remove the cable from the carb slide. You should be able to pull and push the cable end and move the throttle tube freely by doing it. If you can't then remove the throttle tube and lube the cable, clean the inside of the tube and the handlebar and put it back together with no oil on the utbe or bar.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Kernowskinz89]

So I removed the filter pod and looked into the carb head whilst gently moving the throttle to visually look at the movement, it snapped back into place / snapped shut again smoothly and with ease.
I then checked the spark plug which was working albeit very charred and black but dry.
Then I proceeded to remove the whole carb and look into the ports and reservoir for any grime or dirt/debris.
This was my mistake as I haven’t had it ignition since, despite priming.

Currently my battery is flat and on the charger.
I must have tweaked something somewhere and now the settings are off/out of sync.

Dear oh dear
Feel like such a fool, at least before she was working even with high revs
____________________
Adrenaline has a colour… and it’s brown.

[Pete.]

If you have had the carb off and now it won't start check that the cable is sat down properly in the adjuster on the cabr top. Sometimes they pull out and catch on the adjuster lip then the slide won't go down properly and the engine won't start.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Kernowskinz89]

[Kernowskinz89]

Also my apologies for the bombardment but I think the air mixture screw is way off but I’m not certain at all where it should be setting wise.
____________________
Adrenaline has a colour… and it’s brown.

[Mysteriass]

Inside the carb, there's a valve that's operated by the float.

I'm thinking maybe the valve is sealing OK (or not) but that the valve seat itself is leaking due to a dessicated washer. That might cause an "overrev" by letting in too much fuel.

Get a full carb kit either way and rebuild the whole thing including that valve seat washer (assuming it's got one).

Another trick to checking leaking perished rubbers is to get the bike running and spray something like brake cleaner onto the rubbers. The bike will probably rev up if it ingests the aerosol through a breach in the rubber.

Rule of thumb with the air mixture which may or may not be correct in your case : turn it all the way in then turn it back out twice.

[Pete.]

Have you had the top un-screwed? A common mistake is to put the slide in wrong. One side of the slide there is a slot that locates with a small pin in the hole, the other is an angled face that contacts the throttle stop screw (the screw with the spring on it). If you put it in backwards the slide only goes half way down.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Kernowskinz89]

[stinkwheel]

If you've moved it from where it was and didn't write down the position first, reset it to factory settings and go from there. They are usually very close to correct, especially when it comes to idle settings.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Kernowskinz89]

[Kernowskinz89]

Ok, so todays plan is drain the reservoir and remove the carb again and try to reset the whole system.
I’ve been trying to look into what factory settings are on a Mikuni slide carb to no avail or just a carb for that matter but obviously every carburettor is different.
I need to at least try to correct my wrong on the air mixture screw and start there.
If I can get it running again at least I can adjust whilst running.
Thank you to all for your help so far.
Updates to come.
____________________
Adrenaline has a colour… and it’s brown.

[stinkwheel]

You need a workshop manual for your specific bike. That will have all the standard factory settings in there
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Kernowskinz89]

Update [success]
In short, after draining the reservoir and cleaning the whole lot with brake cleaner, I started with a freshly charged battery (ran it down trying to turn over the engine yesterday)
I started with the pilot screw all the way in (seated? But not forcefully) retreating half a turn after a short go on the ignition each time.
With that approach and the help of my Haynes manual I got her running and slowly, sensibly adjusted until the engine idled at around 1200 revs or just over. Very calm sounding.
Also as a bonus, the mystery revving in neutral is gone (for now)

Thank you for all your help guys.
There’s a lesson to be learnt here (don’t mess with your carb)

A happy ending thank god, a working bike too.
I also adjusted my clutch action too as it was very out/loose.
Better action on my clutch lever now which has improved my clutch control when coasting up to lights enormously.

Once again an enormous thank you from this rookie.
____________________
Adrenaline has a colour… and it’s brown.

[to v or not to v]

now get out and enjoy yourself before the weather changes
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current bike Yamaha Thunderace.
Moto Guzzi V7.