GS125 carburettor problem

[Propellor]

Bike in question is a suzuki GS125 so I guess it qualifies as a thumper, so I'll float this question here if I may?

A mate and I were working on a problem with his sons GS125. We think we know what the problem is. The symptom is that the engine revs linger for a few seconds once the twist grip is rolled shut. We have had the air box off and can clearly see that the slide is definitely at bottom of stroke but the engine revs on for a few seconds at around 4k revs before finally settling to tick over. So the problem doesn't seem to be mechanical. We reckon there is an air leak at the carb to head joint. This looks dodgy and we reckon the symptoms fit.

First off, does anyone agree? Secondly and the main reason for this post, can anyone explain exactly how the principle works? An engine obviously needs air to go faster but it also need fuel, so where is it getting the "rogue" fuel from? I can only assume the pilot drilling into the Venturi? But this seems too far upstream for the "rogue" airflow to pass over?

So what exactly happens? Anyone know?

Cheers.

[JOOLZ]

Yes it could be an air leak or maybe the idle mixture screw is set a little lean

[500 T C]

just a thought ,is the  needle moving with the air slide?Might be worth a look.

[Propellor]

Thanks guys.

The carb is a new (3000miles use so far) mikuni. Made in Japan. We've had it apart twice and everything is in order. With the airbox removed we can see that the slide is fully down, yet the revs hold on longer. But as you say, we wouldn't definitively know the needle has returned. What we will do is replace the rubber "manifold" because that looks dodgy. Then if the problem persists we will take on board the comment regarding the needle and try to fathom if it is pushing up somehow. We can already get the needle to lodge onto the step caused by main jet, but only with the needle badly askew in the emulsion tube. When "dangling" vertical everything appears fine. The other thing is that the bike ran fine before winter but badly after a few months stood, so I guess I should have told you that in the first place!

The thing I was unable to fully explain with the manifold air leak scenario, is what exactly happens? How/where does the "rogue" airflow get the petrol it needs? And once it has it, why do the revs eventually fall to tick over?

Cheers.

[JOOLZ]

The thing I was unable to fully explain with the manifold air leak scenario, is what exactly happens? How/where does the "rogue" airflow get the petrol it needs? And once it has it, why do the revs eventually fall to tick over?

The rogue airflow does not get any extra petrol, it leans the engine out.
I suggest you have a read up on carburetor theory its all fairly straight forward and can be broken down to a few main pointers,
idle system, slide cut away,  needle clip position, needle type and finally main jet. All of these overlap slightly.
Your problem is that it is running lean thats why the revs hang on and take a while to settle back to tickover
so check that the inlet rubber is sound and not leaking, if that is ok than you need to richen up on the idle mixture screw and readjust the idle speed.
Modern petrol has a lot of other chemicals in it including 5% ethanol/methanol so any carb settings given in manuals are only a guide and you should always err on the side of richness

[Propellor]

The thing I was unable to fully explain with the manifold air leak scenario, is what exactly happens? How/where does the "rogue" airflow get the petrol it needs? And once it has it, why do the revs eventually fall to tick over?

The rogue airflow does not get any extra petrol, it leans the engine out.
I suggest you have a read up on carburetor theory its all fairly straight forward and can be broken down to a few main pointers,
idle system, slide cut away,  needle clip position, needle type and finally main jet. All of these overlap slightly.
Your problem is that it is running lean thats why the revs hang on and take a while to settle back to tickover
so check that the inlet rubber is sound and not leaking, if that is ok than you need to richen up on the idle mixture screw and readjust the idle speed.
Modern petrol has a lot of other chemicals in it including 5% ethanol/methanol so any carb settings given in manuals are only a guide and you should always err on the side of richness

I thought I already broadly understood the basic theory, albeit strictly as an interested lay person. I'm not doubting that an air leak is almost certain to be the root cause, but I'm struggling to convince myself that there is not some "rogue" petrol involved as a consequence of the leak.

You've been courteous and interested enough to offer advice so let me tell you a bit more information.

This thing hangs on at 5000rpm after the throttle is shut, and takes about 3 or 4 seconds to drop to tick over speed. Would simply a leaned out mixture explain those revs? There's another factor which I've only just realised is a clue. We screwed the throttle stop screw right out so the slide was at rock bottom. Like this the problem all but disappeared. Certainly a lot more like you'd expect, but the thing obviously just stalled each time. To see if it was the throttle stop screw jamming the slide, we adjusted the throttle cable to get the slide to "hang" on the cable at approx the right height. The problem came back.

There is strong circumstantial evidence to me, that rogue petrol is being drawn up through the needle jet when the throttle stop screw is at the correct position for normal tick over.

Even if that is what is actually happening, I'm still not able to offer a conniving explanation of HOW. I'm a bit sad in that, as well as finding a solution to a problem, I'd like to know the logic or explanation behind it.

[Propellor]

..........I suggest you have a read up on carburetor theory........

I'm doing it!

I have another idea which I'll float (hee hee) past you.

Forgetting the air leak for the moment. What would happen if the air jet feeding air into the chamber surrounding the emulsion tube got blocked? As well as cancelling out the main reason the air jet and emulsion tube are there, how about the following....?

Would this chamber fill with un-emulsified petrol? If it did would this create a higher mini reservoir of petrol? Bearing in mind that the height of the column of fuel is fundamental to whether fuel gets drawn out or not (depending also on the pressure drop), would this possibly create the symptoms we have?

Ok, the above is a long shot, but it proves I'm trying my best!

[Steve H]

When you close the throttle you do two things, restrict the air flow into the engine and reduce the amount of fuel able to flow through the needle jet to reflect the size of the throttle opening.  At this point the fuel through the needle jet is effected by two things, 1) the normal vacuum due to Benoulli theorum and 2) a huge vacuum caused by the engine acting as a big pump running at 5000 rpm trying to drag in air. In this case 2) will probabaly dominate and the mixture will be rich. If you throw an air leak into the equation this will mean that the rich fuel mixture can be burnt by the extra air causing the revs to 'hang' on.
The purpose of the emulsion tube is to weaken the mixture at large throttle openings, if the emulsion tube is blocked or the air jet blocked the mixture will be rich at higher speeds and you will almots certainly have noticed this by the engine stuttering.

[Propellor]

When you close the throttle you do two things, restrict the air flow into the engine and reduce the amount of fuel able to flow through the needle jet to reflect the size of the throttle opening.  At this point the fuel through the needle jet is effected by two things, 1) the normal vacuum due to Benoulli theorum and 2) a huge vacuum caused by the engine acting as a big pump running at 5000 rpm trying to drag in air. In this case 2) will probabaly dominate and the mixture will be rich. If you throw an air leak into the equation this will mean that the rich fuel mixture can be burnt by the extra air causing the revs to 'hang' on.
The purpose of the emulsion tube is to weaken the mixture at large throttle openings, if the emulsion tube is blocked or the air jet blocked the mixture will be rich at higher speeds and you will almots certainly have noticed this by the engine stuttering.

Thank you.

That makes a lot of sense to me. Given the scenario of closing the throttle from high rpm, the Bernoulli effect will be low as it relies on air speed, yes? But the engine is pumping away trying to turn air over at a fair rate, pulling against an almost fully closed inlet. You're saying that in this condition the mixture is very rich. That makes sense because there's no air (in any great volume) but still a strong pressure difference between float chamber and intake throat. The combustion chamber can't burn the mixture fully because the fuel/air mixture is all wrong. But add in some rogue air and suddenly the fuel/air mixture is about right again. And enough air volume to keep the engine revving. That right?

Our test of screwing back the throttle stop indicates that it is pulling it up through the needle jet.

What happens in the three or four seconds to allow it to eventually settle to normal tick over?

Re the air jet/emulsion chamber, I understood pretty much what that does, but was looking for long shots in the absence of being convinced that a lean mixture could cause such a dramatic effect.

I've done a lot of searching and loads of sites say it leans out the mixture but none I've found give any kind of (scientific) explanation. Of course (I assume ) that with the air leak most other engine conditions will result in a lean mixture?

Thanks again.

[johnr]

but the emulsion tube and the needle that sits in it are only effective when the throttle is actually open and the slide is up. when the throttle is shut then the engine is fed a tiny amount of fuel by the pilot air setting , a has been said an air leak will let the revs rise but the engine wont produce any power due to it being massively lean, as a test to prove your air leak theory, start the engine and without touching the throttle with the engine running, spray wd40 onto the inlet stub, if its leaking it will draw the wd into the engine with the air. and the engine will rev higher.

[Propellor]

but the emulsion tube and the needle that sits in it are only effective when the throttle is actually open and the slide is up. when the throttle is shut then the engine is fed a tiny amount of fuel by the pilot air setting , a has been said an air leak will let the revs rise but the engine wont produce any power due to it being massively lean, as a test to prove your air leak theory, start the engine and without touching the throttle with the engine running, spray wd40 onto the inlet stub, if its leaking it will draw the wd into the engine with the air. and the engine will rev higher.

Thanks.

I'm not keen on my own mini theory about a blocked emulsion chamber air jet. It was a long shot. Grasping wildly at straws if I'm honest! Where I think it falls down is that a blocked air jet feeding the emulsion chamber would in itself prevent atmospheric pressure from acting on the fuel therein. A fundamental requirement if I understand things half right.

The explanation offered by steveH sounds pretty good to me, based on my understanding of things. It seems to make sense. Do you think?

It does seem as though fuel can be drawn up through the needle jet, under certain engine conditions, even with the slide at tick over height. We appear to have experienced it with our particular problem.

Cheers

[Propellor]

The word back from my mate is that a new inlet rubber/manifold has not cured the problem.

He's going to double check carefully throttle cable and needle seating but, to be honest, I really think they're ok.

They recently put a new Chinese made cylinder head on. Is it possible air can get down the inlet valve guide/stem gap?

Cheers.