What does the float height actually affect?

[Lord Percy]

Still trying to get to the bottom of what I am now certain is a carburettor issue, as I've replaced everything else apart from the engine! Well... actually I've replaced the carburettor too, but I just don't know about how it should be set.

The problem is that splutters at the top end of the engine. Anything above about 7000revs, and at wide open throttle, causes the bike to just not want to do any more.

After much deep thinking, while riding and getting a 'sense' of what was happening, where I spent too much with my thoughts in my head instead of focussing on the road I've concluded that the engine simply isn't getting enough fuel, and ONLY when the throttle is almost wide open, and ONLY when the revs are pretty damn high.

Currently my bike runs as so - Idling fine, low revs fine, mid revs loads of power!, highest revs immediate power loss, loads of spluttering and can't push it any further (45-50mph average - it's just a GN125 but that's still terrible, as it's rated for 60+mph and is classed as usable for the Mod2 test). It seems to me that all the fuel is going at the mid revs and there's none left to give after that.

The fuel line is fine. I have an inline filter which clearly shows it flowing quite quickly into the carb when I flick the tap to open in the morning.

Which I suppose means the carburettor isn't set right. I've tried every setting for the diaphragm pin, and it seems to be pretty optimal at the moment. (other settings made everything a lot worse). My understanding is that a lower pin means less fuel for low revs, and a higher pin means more fuel in general.

The one thing I haven't checked, and don't even understand, is the whole thing to do with the float height. What does it mean? Am I right in thinking that the floats are used to determine how much fuel is stored in the float bowl, and therefore how much fuel the carburettor can provide to the engine at any one time?

Either way, I'm totally waffling now, and, regardless of my own personal motorcycle problems, my main point still stands alone - what does the float height actually do?

What happens if you raise the float height?

And what happens if you lower it?

Cheers!

[StevRS]

Reasonable thread here: https://www.advrider.com/forums/showthread.php?t=110771
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MV Agusta Brutale 800 RR, Unfinished 1978 XS250

[stinkwheel]

Raise float height- carb overflows
Lower float height - bike can run out of fuel under high demand.

The float height can't alter itself and is not adjustable on most bikes anyway.

Check it, don't play with it unless you measured it as being wrong.

A lack of fuel flow due to a blocked filter/tap/breather would be more likely but remember that 9 out of 10 carburettion problems are actually electrical.
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“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.

[Lord Percy]

[stinkwheel]

Selective reading and grasping at straws.

I also said float height doesn't alter itself, that you should measure it to see, that a blocked tap/filter/breather would cause the same symptoms and that 9/10 apparent carb problems are really electrical.

The chances of the float height having mysteriously altered itself in TWO carburettors are slim to non-existant.

Fiddling with your float height is a slippery slope, will leave you deeply frustrated and is tricky to put back how it was. Take it from me, it's not your float height.

You wont but I feel better having told you.

Any motorcycle I've ever owned has run acceptably well on factory carb settings. Any fiddling is just fine tuning/tweaking.
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“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.

[Lord Percy]

Sorry, I forgot to mention my bike didn't do this until I stuck the new carb in. I originally replaced the carb because it was idling high and I hadn't a clue what to do since I'd only owned the bike about 10 days, so I figured a new carb for 30 quid would be worth it.

Anyway I should also mention that this problem happens more when I'm going uphill, which to me again points to the float bowl because on a hill the fuel is at a different angle so the floats will sit a bit differently.

And, yes, I certainly will be playing with the float height very soon

I'd like to learn anyway. Trial and error n' all that.

[KDub]

Replace spark plug if you haven't already, check the HT lead for splits and replace it if you find any. Check the main earth point/points (I think yours will only have the one but I can't quite remember) and ensure you've got good metal to metal contact. Finally take the coil off the frame and again ensure you've got good metal to metal contact.

Then get a multimeter and see what voltage you've got from the battery to the coil whilst the bike is running; if it's less than 12 then you have a problem. A good manual should also tell you how to check that your coil is working well.

These bikes are sensitive to good earthing points, if they aren't right they can cause all sorts of weird and wonderful problems.

I had one, and it used to indicate left when I applied the rear brake. Turns out it was a crap earth.

If none of that works, and you don't know the service history of the bike, then it could well be valve clearances.

[Fizzer Thou]

The float height adjustment will mostly affect only part throttle openings.It has a static setting that,when at idle,the float valve will open only very small amounts.At WOT (Wide-Open-Throttle) the float will be at its lowest for full fuel flow,so the height adjustment will not affect WOT.
But,if the needle has been raised,then this could be causing a rich mixture when the main jet is at its most effective,as in from 3/4 throttle to WOT.But if the needle is not adjustable and the main jet size is standard,then there must be a problem elsewhere.

Is the carb diaphragm seated properly?The best way to check is to remove the carb top screws and the carb top and to ensure that the spring is seated correctly in its guide.Then,lift the slide by hand through the inlet side of the carb mouth.Hold the slide as if in its WOT position,then lower the diaphragm down into its seated position,all of the time holding the slide up.Once the carb top is in place then relocate the screws.Once tightened down,the throttle slide should rise and fall against slight resistance.

If the diaphragm is split then this can cause the slide,which lifts depending on revs,to not lift to its fullest extent.
Check also that the main jet holder has no blocked bleed holes in it.
Check also that if there is a plastic guide around the,which looks a bit like a funnel with notches cut where it locates on the main jet tower,is located correctly.A picture would help if you can put one up
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Just talk bikes.What else is there?

Always have a 'Plan B'

[Lord Percy]

The carburettor is a CV one, so it doesn't have that sliding thing. I've checked and rechecked the diaphragm and everything anyway. One thing that makes me suspicious of it being a carb problem is that the spring wasn't even in place when it came out of the box. I had to open the thing up and redo it myself - and that's how it came from the factory! So I don't have much faith in anything else.

Also, my manual says the diaphragm needle circlip should be two notches from the top, whereas in this carb I bought (apparently preset for GN125s) the clip was four from the top. I put the needle where it was supposed to go and the bike would hardly rev above 2000. So now I've tried every needle position possible and it seems best where it was originally, at the clip 4th from the top. I also noticed the needle in this new carb is a tiny bit different to the diaphragm needle in my old carb.

Basically... I'm going with the option of ignoring the book and figuring the whole thing out myself because this carb I bought is definitely not set how it's supposed to be. At all.

I'm gonna tweak the float height. If it doesn't work, I'll touch it back to where it was.


I've changed the spark plug, checked it and rechecked it, and had the HT lead off and checked for a good connection (got a nice shock in my hand a few times )

Good shout on the earthing thing though. Problem is I haven't a clue how to check it, or where, or anything. The Haynes manual has a wring diagram for my bike but I haven't a clue where to look. I can't find anything labelled as 'earth'. However on the diagram some of the components have an arrow leading away from them to nothing. I'm assuming this might mean they're earthed? The components are the HT coil, Neutral switch, Start motor, and Battery. Even though, if I'm right that the arrow means they're earthed, I still don't know what to look for... will there be a wire leading away from them onto the frame or something??

[KDub]

The coil, neutral switch, and starter motor are earthed to the frame by their mounting hardware, or straight through the component itself. For example, if you take the coil off it will have a sort of brass/copper colour to it underneath the bolt, and the frame won't have any paint on it. The battery is earthed to the frame or the engine by a wire, usually black, that you might be able to follow through to where it attaches to either the frame or the engine.

[Fizzer Thou]

A CV carb does indeed have one of those 'slidey' things

https://www.twowheelsforever.com/how-tos/keihin_40mm_cv_carburetor_schematic_diagram.html

and is attached to the diaphragm and it is what the needle sits in.

If you have tried to ride the bike with WOT,what colour does the spark plug come out looking like - black and sooty would indicate a too rich situation,as if the main jet had fallen out of its location,or white and blistered,as though the inlet manifold was leaking air past the seals and causing the engine to run much too lean if at all.

If you have a multimeter,measure the spark plug cap resistance,which should be teween 5-10k ohms.

Is the air filter clean?Is it a standard one or a foam replacement item instead of a paper filter?

Has the spark plug got the correct gap?(usually around 25' gap)

The earth wires are those that are connected to the negative post of the battery via the frame as a return route.Sometimes these are depicted on a wiring diagram as a point going downwards as if to earth.
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Just talk bikes.What else is there?

Always have a 'Plan B'

[Lord Percy]

Right then, that's good to know.

I haven't mucked with the float heights per se, but there was one bit that looked unnaturally bent, so the floats would rest a bit higher than they maybe ought to have done, when the chamber is empty. By bending this bit, in theory the float valve will open a tiny bit more when the float chamber is emptier. Hard to explain and a photo wouldn't show what I've done because the change is so minute, and not a direct alteration of 'float height'.

Anyway, it's a small effort, and I'll try to rummage around all the earthed parts to see if they're connected properly.

Last night while riding for 2 hours back to Reading from London, the lighting in the clocks gave out as well. The rev counter was unlit from the moment I started the journey, then the speedo light stopped after about an hour. I wonder if that issue is connected, if it is, as people say, to do with the electrics...?


The air filter was replaced a few days ago. And yep ok I understand what was meant by the sliding bit now

The spark plug was coming out all burnt before I put in this new carb. Now the spark plug is a totally fine every time I check it. The plug gap is fine too, as are the valve clearances.

I really wish I could get my hands on one of these multimeters because it's one thing I haven't been able to check yet, the actual power, resistance, voltage and all that.

[stinkwheel]

[Lord Percy]

Right then, will do I think.

Anyway, I just had it out on the road after bending that little bit. As expected, no change.

Also I checked the earthing of the the HT coil and battery - both seemed fine. I had a little scrub around the connections to the frame, so I'm certain there's contact.

On the journey I came to a spluttering stop at some traffic lights. I thought I'd really fucked it, waved all the traffic past when the light turned green, and then realised it had gone to reserve

Anyway nope, no change at all.

One thing I notice is that it has absolutely shitloads of accelerating power when the engine gets to around 6000 revs.

What happens is, it accelerates at what I would consider to be a normal rate (maybe it's a low rate?), and then suddenly, at 6000rpm (or thereabouts), it clearly gets more power, to the extent that I really feel it pulling forwards, and then at about 8000rpm it starts the spluttering thing and absolutely will not go faster than 50-55mph, in 4th gear. 5th gear does fuck all, I assume because in 5th gear the engine revs go lower than that point at 6000 where all the power seems to be.

I wonder if it's still to do with the setting of the needle in the diaphragm? Maybe fuel is being provided at the wrong points?

Is the shape of the diaphragm needle important? The needle in this carb is clearly different to the one in my old carb. I'd show photos but my old carb is at my parents house 200 miles away.

I'm trying to rule out any kind of fuel blockage because this problem is permanent and always happens at the same point (whereas a fuel issue would surely be intermittent and cause problems across the whole range of running).

[jimspeed]

Have you tried swapping the old carb back on? Seems the logical step to me.
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Hyosung cruise 125(passed test on, sold) Kawasaki el 252 (better than expected but sold on) Kawasaki GPZ500S first "big"bike.(sold) ZZR600 E5..Z750 2007,ER5, currently on a 2008 Enfield bullet electra x and loving it..
,"Alpha-9: Is there any correlation between dyno rod and dyno kits?"

[Polarbear]

If this was a bike with points I would be checking them, and the advance and retard. (yes I'm old enough to use a fag paper to set the points )

If you have tried with two separate carbs and the faults are exactly the same it lends credence that the fault is somewhere else and as stinkwheel said, electrics are the favourite.
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Triumph Trophy Launch Edition

[YBR Ric]

The new Carb most likely has a smaller main jet than your old one did.

[Lord Percy]