SL125 carb issue

[Laava]

I have a std carb that I cleaned meticulously and put a kit in, all new jets gaskets etc. i have adjusted the float level to what it says in the manual, which is a bit ambiguous, but basically set it at where it seems right and it runs as if, to my limited experience, the idle circuit is not working. It basically will only idle with the idle screw in as far as it will go, at which point it is probabaly idling on the main jet only. Then if I turn the fuel off it will get to a point where it starts to rev up a bit and then if you give any throttle the revs will hang high and very shortly die altogether as the fuel gets too low.
So I have tried it running on about 4 different settings with basically the same result. I have used the clear hose method as well with the fuel level anywhere from 3-0mm below gasket surface.
OK, so I have tried a different carb, as I have 2 bikes the same, a brand new chinese one on the bike and it ran perfect. The original carb ran the same way on the other bike so I am convinced it is neither fuel or ignition. Any ideas or should I just give up and buy another chinese carb? Seems a shame as I was hoping to keep it as original as possible. Thanks in advance, Al

[F5 Dave]

So you have compressed air? When you remove the pilot jet and blow through, does it spray out into the throat? It is serious tiny. You may have to block the air jet at face of carb as there 3 holes to create 2 hole thus pressure in and out.

[Laava]

I threw the carb into a water tank for a month before I started and have run carb cleaner and compressed air thru it multiple times. It seems my options now are try what you suggested, or replacement. But what you said makes sense…bit late now, don't want to wake the missus up!

[husaberg]

I threw the carb into a water tank for a month before I started and have run carb cleaner and compressed air thru it multiple times. It seems my options now are try what you suggested, or replacement. But what you said makes sense…bit late now, don't want to wake the missus up!

If you are quick you can get most of the way through before she wakes up.........but on the subject of the carb I am with Dave use compressed air on everything you can also use plastics brush bristles out of a gong bush to push into the carbs crevice's on most jets and orifices.

.
I would also confirm the new kit parts are the right parts.

The hanging throttle normally means pilot circuit is too lean. but if you are turning off the fuel that's to be expected.

if its too rich it will start too easy from cold sans choke.
it will blubber. in response to thottle
make sure your needle is in the correct position as that can be ambiguous so put it in the middle

Honda float levels are set like this

https://www.hondatwins.net/attachments/floatadjustment-jpg.40872/

i have heard of new float needles never working but you will have fuel leaking out normally

pm me you address i have a CB100 or sl carb somewhere and some bits and bobs.
i guess the spare SL can be modified and tuned?

[Laava]

Thanks Husa, will work thru that lot. I did check all the jet sizes when I rebuilt the carb a couple months ago and did a static needle valve check with a bowl full of fuel before fitting. The accepted norm with the clear tube check seems to be 1-2mm below gasket mating surface of carb body. The workshop manual shows the carb being on it's side but following that diagram, the float is hanging freely not even in contact with the needle valve and cannot be adjusted. I did it upside down which makes more sense. Is frustrating me as the carb really couldn't be any more basic. I hope it is just an insanely stubborn blockage!

[husaberg]

Thanks Husa, will work thru that lot. I did check all the jet sizes when I rebuilt the carb a couple months ago and did a static needle valve check with a bowl full of fuel before fitting. The accepted norm with the clear tube check seems to be 1-2mm below gasket mating surface of carb body. The workshop manual shows the carb being on it's side but following that diagram, the float is hanging freely not even in contact with the needle valve and cannot be adjusted. I did it upside down which makes more sense. Is frustrating me as the carb really couldn't be any more basic. I hope it is just an insanely stubborn blockage!

My experience is you measure the float with it sitting on a 90 angle. Maybe i have always done it wrong?

[Laava]

My experience is you measure the float with it sitting on a 90 angle. Maybe i have always done it wrong?
If you mean with the carb upside down, then that is what I have done and it makes the most sense. Maybe the manual is just a bad translation…

[F5 Dave]

If you spray some brake cleaner in the pilot cct, block the airjet entry and blow air through it should blow the spray out to the throat where you can see it (through safety glasses) . No pathway, no idle cct.

[Laava]

Right so the bowl gasket in the rebuild kit is non petrol resistant.
The passages are all clear, I am unsure why this is an issue, I am going to try to lower the fuel level a mm at a time, aware that there is a point where it becomes so low that the float is basically touching the bowl so no petrol can come in at all.
The photo shows it with the original, prob 50yr old, gasket. Which I am back to using...
Also showing the carb upside down with the vernier sitting in the notched sections. My understanding is that it is always measured from gasket mating surface unless it has these notches?

[Laava]

My other thought was that the fuel level setting is predominantly for the main jet right? And on all the bikes I have had, they will continue to idle way after you have turned the fuel tap off. So it has me flumoxed, it should be working, even if badly, but no.

[F5 Dave]

You should be able to remove the main jet and not tell the difference until you are at about 1/2 throttle.

[F5 Dave]

You've tried another carb so the likelihood of an airleak is low. But think about the interface.

[Laava]

Is not an airleak, I sprayed start ya bastard about to check, is not a choked up air filter, is not ignition, I have spent 3 hrs on it today and got nowhere. Unbelievable. I am just going to buy another chinese carb, although I will interchange with the one I got just to check…but it ran and rode perfectly with it(chinese) the other day…does my fucking head in this kind of shit
I have bent the float tab so many times now trying different levels, it must be suffering from fatigue!
I have an older shittier Mikuni that I could quickly clean up and put on just to try but I might be just wasting more time

[Black Knight]

Hi Al-I have a the same or very similar carb on one of my bikes,Honda S90,I have had it on and off the bike so many times I can now do it blindfolded.
I have found that the overseas kits can have the wrong idle screw,too short and have no effect on the idle revs-The correct length is 32mm-If you look into the carb at the intake end you can see the throttle valve,it has an angled slot on the RH side which the idle screw goes into.With the correct size screw you will note that as you screw this in it raises the throttle valve.At rest and with the throttle cable adjusted correctly,the gap at the bottom of the valve is around 3mm.Once started and warmed up you can then play with the screw to get a decent idle.
You can now play with the idle air screw to adjust the fuel ratio once you have a decent idle-my base is one and one eighth turns out-in for richer out for leaner-until the motor runs smooth,do this adjustment gently.
I needed a new idle jet,my bike has #38 which is tiny and easily blocked,these are unobtainable but Botany Honda has a close aftermarket which i got and cut 7mm off the length.
If you need a correct length idle screw I have a spare here you can have if you want a quick ride up the back road to collect.
Cheers-Chris

[Laava]

Hi Al-I have a the same or very similar carb on one of my bikes,Honda S90,I have had it on and off the bike so many times I can now do it blindfolded.
I have found that the overseas kits can have the wrong idle screw,too short and have no effect on the idle revs-The correct length is 32mm-If you look into the carb at the intake end you can see the throttle valve,it has an angled slot on the RH side which the idle screw goes into.With the correct size screw you will note that as you screw this in it raises the throttle valve.At rest and with the throttle cable adjusted correctly,the gap at the bottom of the valve is around 3mm.Once started and warmed up you can then play with the screw to get a decent idle.
You can now play with the idle air screw to adjust the fuel ratio once you have a decent idle-my base is one and one eighth turns out-in for richer out for leaner-until the motor runs smooth,do this adjustment gently.
I needed a new idle jet,my bike has #38 which is tiny and easily blocked,these are unobtainable but Botany Honda has a close aftermarket which i got and cut 7mm off the length.
If you need a correct length idle screw I have a spare here you can have if you want a quick ride up the back road to collect.
Cheers-Chris
That is bizarre! I will check that next time I am inspired to go into the garage! That would certainly explain why it started well on a part throttle too…cheers Chris!

[husaberg]

According to this i am doing it wrong?
351617
i always brought it around another 45 degrees to the clockwise.....
Edit Grrr

okay i relented here is the original honda service manual which is how i was taught.
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click it three times.
that bit on top is the bellmouth end.

[husaberg]

Is not an airleak, I sprayed start ya bastard about to check, is not a choked up air filter, is not ignition, I have spent 3 hrs on it today and got nowhere. Unbelievable. I am just going to buy another chinese carb, although I will interchange with the one I got just to check…but it ran and rode perfectly with it(chinese) the other day…does my fucking head in this kind of shit
I have bent the float tab so many times now trying different levels, it must be suffering from fatigue!
I have an older shittier Mikuni that I could quickly clean up and put on just to try but I might be just wasting more time

I am pretty sure there is/was a decent one at my Mums
that protruding gasket could have been holding the float open or closed or both.

[Laava]

I am pretty sure there is/was a decent one at my Mums
that protruding gasket could have been holding the float open or closed or both.
I wondered about the protruding gasket thing too but it did the same with the old fibre one in. Funnily enough, I think BlackKnight has made an interesting observation about the idle screw replacement simply being too short for the job, which is something I never would have thought of and also a rookie mistake at the same time!😁😁😁
the link you posted earlier Husa showed the carb simply being upside down. But the initial contact method you are talking about is sometimes a thing too. All the utube vids show the carbs as just upside down. In any event, I think my problem hasn't fully been float level. I am going to check the chinese one to see it's float height. Assuming it has the same or similar internals. The outside is identical.

[husaberg]

I wondered about the protruding gasket thing too but it did the same with the old fibre one in. Funnily enough, I think BlackKnight has made an interesting observation about the idle screw replacement simply being too short for the job, which is something I never would have thought of and also a rookie mistake at the same time!������
the link you posted earlier Husa showed the carb simply being upside down. But the initial contact method you are talking about is sometimes a thing too. All the utube vids show the carbs as just upside down. In any event, I think my problem hasn't fully been float level. I am going to check the chinese one to see it's float height. Assuming it has the same or similar internals. The outside is identical.

some what people refer to as idle screws are a tapered screw that only move the slide up. Too short and it will not move anything
Others are a pilot air bleed.
Some are a fuel screw.
if any are wrong they will not work.
i can remember a bike that would only run for a few minutes then stop it was the fuel tank breather blocked.
another that would not rev past half throttle all of a sudden.... that was the main jet falling out.
make sure you have the choke set properly too. i have seen them set off for on. Or flapping in the breeze.

[Laava]

Yeah, well I went and checked the idle speed adjustment screw and as BlackKnight said, it seems short and is only 26mm long and not moving the slide up by much at all when fully in. I have a couple or three spare carbs in a box so I will sort thru and see if I can find one, or when my covid has gone, go for a ride to visit Chris up north!😁
Thanks everyone for all your input, I am still hopeful that I get to keep my 50yr old original carb!

[Laava]

Just again, wrt to float measurement, I am sure I remember my 450 honda manual stating that the measurement is taken from the notched area in the skirt around the gasket mating surface. Which is the same on this carb as well. Am I right about that?

[Black Knight]

Just a thought Al,seeing your throttle screw is wrong size,maybe also the air screw is incorrect-should be 18mm long and I would think the taper might be important,I have a spare air jet here.-The correct adjustment of the air jet is critical to smooth running once the idle speed is sorted,the slightest turn in or out will effect the running, so pretty delicate.

[Laava]

Thanks Chris, but after some careful investigation, involving inspection of the identical chinese carb, here is what I found,
1, The float height is as Husa described. ie. put the carb upside down and then revolve until the float starts to swing away from the needle. Right at that point, touching but not compressed the needle valve spring, measure the float height. And it is to the apparent notches, not the gasket surface. So, at 24mm, when you then invert the carb, it compresses a further 3mm or so which is a lot and very misleading if you try to adjust from this position
2, the throttle screw is only short by 1 mm or so and in any case would in theory still work. I have 5 of them including the new one from kit. And they are mostly 27mm.
3, on the chinese carb, which is running fine, the gap under the throttle slide on the front is only about 1 mm.
4, it is apparent I need a new float bowl gasket as I can't be arsed trying to make one and the replacement is not a petrol grade material

so, I have not test run this setup so can't comment as I now need a new float bowl gasket!
thank fuck I still have a bike I can ride and now I need to do that!

[Black Knight]

Fair enough-I often wonder about float clearances and how accurate these need to be-after all you are trying to achieve (a) sufficient fuel in the bowl to reach the jets and (b) adequate shut off at the float valve to ensure bowl does not flood,or too much where the float needle jambs in the valve-maybe that is where some form of accuracy comes in.
Years ago I saw a simple metal gauge that had "shoulders" cut into a piece of flat steel,the distance from the top of the gauge down to the top of the shoulders was the required mm,you simply held the carb upright,placed the shoulders under the floats and the top of the gauge under the carb where the bowl gasket sits then bend the tab until the seated height of the floats is even with the shoulders on the gauge.Would be pretty easy to make one and would save a shitload of farting about.
I buy a lot Honda spares from Malcolm at Anaconda (was Econohonda) in Te Aroha.If he does not have the item in stock he places fairly regular orders with CML and as a result you do not get stung with the high freight cost on an individual item from CML.

[Black Knight]

Actually that pic that Husa posted looks very much like the gauge I referred to but it appears to be adjustable to suit varying float heights-I am now wondering if the local Honda dealers workshop would have one of these,might be worth an ask.

[Laava]

I think I did myself a disservice watching videos on youtube. Without exception they showed the carbs being inverted, which still makes the most sense to me, and the difference from there to recommended is considerable. Anyway, I just need to track down a good quality gasket to make it stop weeping but I can test it all without that…got to go for a good blowout yesterday afternoon on the monster so is allgood!

[Laava]

Right, so I am going to buy another chinese carb. I just cannot get this one to work anywhere near properly. Put the chinese carb on the good bike, I was hoping to keep original, and went for a big, if slightly illegal ride.

[husaberg]

I think I did myself a disservice watching videos on youtube. Without exception they showed the carbs being inverted, which still makes the most sense to me, and the difference from there to recommended is considerable. Anyway, I just need to track down a good quality gasket to make it stop weeping but I can test it all without that…got to go for a good blowout yesterday afternoon on the monster so is allgood!

Mikuni show it needing to be inverted as well
http://www.mikuni.com/pdf/vmmanual.pdf

The Keihin manual describes it quite well
https://www.keihin-na.com/assets/1/7/slide_valve.pdf
the pic is a little misleading as its a Downdraft looking FCR
but it shows it on a 45 degree.
351679

[Laava]

Yep, so I have set it so it is just contacting the valve stem. I rechecked it after removing it and it is still exactly on spec. So is the chinese carb and it runs fine.

[husaberg]

Yep, so I have set it so it is just contacting the valve stem. I rechecked it after removing it and it is still exactly on spec. So is the chinese carb and it runs fine.

Is the o ring on the carb throat and the gaskets on the intake i good nick?

[Laava]

Is the o ring on the carb throat and the gaskets on the intake i good nick?
Yep and spraying ether around there confirmed it. Have just given up now. Have spent more on fixing this carb than a new chinese one costs…

[husaberg]

Yep and spraying ether around there confirmed it. Have just given up now. Have spent more on fixing this carb than a new chinese one costs…

Fair enough......

[Black Knight]

Float height sanity check-Turn on fuel cock to fill bowl then turn off-Drain fuel from carb into a container-Remove bowl and tip the drained fuel into it-should be close to half way up the bowl-adjust tab if required.
Install Chinese carb:psst:

[Laava]

Taiwanese carb ordered, end of issues in theory…

[Laava]

Ok, update time. I bought and fitted a chinese carb. It instantly started and idled but when you open the throttle it dies instantly. The float height is exact the pilot screw is at 1.5 turns out. If you feather the throttle carefully the revs get up and you can ride it but at idle you just cant crack the throttle.
I have a mate who is an old school mechanic and told me he could fix it but no, he had no success either. He tried putting the original, reconditioned carb back on, which is so much worse again. Timing is spot on and has been rechecked so many times, I have tried several spark plugs, have had the coil, condensor and the current spark plug tested professionally as the spark seems weak. The compression is high. The battery is fully charged and short circuiting the bikes wiring loom makes no difference.
So, back to the idle thing. It will sit there idling away ok but does a funny little spit/hiccup thing every now and then. And the instant stalling thing when the throttle is cracked is the main problem. Does that mean it is too rich? Adjusting the pilot screw has not seemed to affect this. Is the fuel level too high?
I just feel like I have to buy another carburetor but it shouldn't be that hard. My mechanic mate is totally stumped as he has spent 2 days trying to get it to work.

[F5 Dave]

So can you swap the carb from your good bike?

I'd be thinking lean.

old needle in good carb?

Bit of spray engine start just as you crack the throttle. Would replicate a throttle pump bigger engine often have.

Spray around manifold as well, looking for airleak or small split.

[Laava]

So can you swap the carb from your good bike?
Only have this one now.

I'd be thinking lean.

old needle in good carb?
Is all brand new, and I don't want to go down the track of swapping chinese needles and jets with the japanese one

Bit of spray engine start just as you crack the throttle. Would replicate a throttle pump bigger engine often have.
Might be worth a crack, have already done the inlet leak check…

Spray around manifold as well, looking for airleak or small split.
To be fair the only thing I havent done is check the airbox and filter since it was installed. But it was all new so…

[F5 Dave]

Soap spray while leakdown testing my 2 strokes regularly found Reed manifold leaks. Inlet valve closed, carb blocked introduced air could be interesting. But spray if engine start all around would achieve same.

[husaberg]

Earlier i posted about the oring on the carb throat but on early Hondas depending on if it is the alloy manifold or the rubber they have another oring's pluss a gasket or two soft gaskets. Plus that plastic heat isolator..
353747

[Laava]

I have sprayed ether around there when it is running. Also, once above idle the engine pulls ok, just not entirely cleanly and this off idle death is doing my head in. I am just going to have to buy another carb and gift it to the new owner if it actually sells on trademe,
Of course, all this endless tinkering has resulted in some paint damage which now needs a careful touch up in one place and some inconsequential scratches to the frame where the seat and tank mount up. More little touch ups…

[HenryDorsetCase]

Heres a stupid question. Is the float needle round in section, and does it have the correct taper - and is it the same as the seat it goes into? Is there some sort of cam somewhere that pulls more cable at the beginning of the stroke than the end - assuming not a CV carb i.e. the throttle cable pulls the slide up.

[Grumph]

Years back I had the same problem on a SL125 I'd converted to a trials bike for a mate. Corrected by checking and adjusting ignition timing - and freeing the sticky
centrifugal advance mechanism. Hesitation was slow ignition advance. Got a timing light ?

[Laava]

Heres a stupid question. Is the float needle round in section, and does it have the correct taper - and is it the same as the seat it goes into? Is there some sort of cam somewhere that pulls more cable at the beginning of the stroke than the end - assuming not a CV carb i.e. the throttle cable pulls the slide up.
All new and cable is directly onto the carb slide.

Years back I had the same problem on a SL125 I'd converted to a trials bike for a mate. Corrected by checking and adjusting ignition timing - and freeing the sticky
centrifugal advance mechanism. Hesitation was slow ignition advance. Got a timing light ?
Yep I have a timing light and hooked it all up to check it and dropped it breaking the flash tube. I didn't want to just "buy" another timing light as I always try and fix but you guessed it, I can buy a new timing light cheaper than I can buy a new flash tube!….
Ignition advance mechanism is new and quite free. So do you think it might be too retarded?

[Grumph]

Ignition is one possibility. Have you richened it up on the airscrew ?

[Laava]

Yep adjusting the airscrew makes little difference. I am back to endlessly faffinfg about with the float bowl trying to stop it leaking. The chineses carb gasket turns the petrol in the bowl a horrible brown colour so I waiting for nzpost to drop my new one in the mailbox. The float is set to 24mm as per the handbook and although that is where it runs best, it seems to just end up leaking and fucking the paint on the crankcases. Another thing to fix. I am going to try going to 24.5 mm to see if that makes it better. My mate who worked at forbes and davies all those years ago reckons the needle valve spring seems weak although new so we shall see. I have had to replace the brand new fuel tap I brought with a brand new fuel tap to stop it seeping, contributing to the endless overflowing dramas.

[pete376403]

All new and cable is directly onto the carb slide.

Yep I have a timing light and hooked it all up to check it and dropped it breaking the flash tube. I didn't want to just "buy" another timing light as I always try and fix but you guessed it, I can buy a new timing light cheaper than I can buy a new flash tube!….
Ignition advance mechanism is new and quite free. So do you think it might be too retarded?

Wont fix your carb as such but https://www.jaycar.co.nz/xenon-flash-tube/p/SL2692

[Laava]

Thanks Pete, I did see that one listed but it is a completely different shape to the optilux one and I know nothing about compatability….

[actungbaby]

I put the slide in wrong way around full bore start.
I got XL 100 k2 if want try
My carb ur welcome .
Bike hasn't gone decade and a bit.well been lazy 353782353783

Sent from my CPH1941 using Tapatalk

[Laava]

Thanks Achtung, would have been keen if I was a bit closer!….

[Laava]

Wont fix your carb as such but https://www.jaycar.co.nz/xenon-flash-tube/p/SL2692
I am going to try and bodge one of these in tomorrow! Plus, if it fails, I can blame you Pete!

[pete376403]

I am going to try and bodge one of these in tomorrow! Plus, if it fails, I can blame you Pete!

IIRC from the mains powered timing light I made years ago, the leads of the tube are not polarity sensitive, and the trigger is single connection that ends up at the plug lead pickup

[Laava]

So, yep, that worked! Of course I did ,make a meal of it! I had dropped the strobe in the first place breaking the flash tube and being of scots heritage, reluctant to spring for a new one! The replacement tube is of course 80% of the price for a whole new timing light. However I took a chance on Petes suggestion at Jaycar for $5.90 and made it fit and it worked on a test on my Ducati. Which I needed to use to get 12v. So then I parked the SL close to the Ducati so I only had to clip the induction lead on the spark plug and we're all set! So while the SL is ticking over I am loosening the cover over the stator to get at the timing marks, and just as it comes loose I notice smoke coming off my exhaust which of course is the induction lead from the timing light. I quickly try to get the lead off the header pipe, turn off the ignition and control the cover coming off in one smooth action which of course does not really happen. The Honda manual says about removing the stator cover to check while the engine is running, "some oil may spray from the engine at this point" and that is an understatement. After a quick clean up, I check the strobe light and find it is back to not working again. I carefully peel back the still warm molten insulation of the outer layer to find a very fine coaxial wire which has melty issues. After a bit of very careful segregation I manage to squeeze in some insulating tape and do a re-test which is successful! So I checked the timing and it was spot on. The new float bowl gasket and increasing the float height by half a mm seem to have resolved all the leak issues and the bike starts and idles but still has the big flat spot off idle. In the meantime, someone has hit buy now on my auction and are now the proud owners of a 51yr old motorcycle!

[F5 Dave]

Just the right buyer required.

Tribulations though. So, the advance mechanism works when revved?

Some ether spray when revved?

[pete376403]

So, yep, that worked! Of course I did ,make a meal of it! I had dropped the strobe in the first place breaking the flash tube and being of scots heritage, reluctant to spring for a new one! The replacement tube is of course 80% of the price for a whole new timing light. However I took a chance on Petes suggestion at Jaycar for $5.90 and made it fit and it worked on a test on my Ducati. Which I needed to use to get 12v. So then I parked the SL close to the Ducati so I only had to clip the induction lead on the spark plug and we're all set! So while the SL is ticking over I am loosening the cover over the stator to get at the timing marks, and just as it comes loose I notice smoke coming off my exhaust which of course is the induction lead from the timing light. I quickly try to get the lead off the header pipe, turn off the ignition and control the cover coming off in one smooth action which of course does not really happen. The Honda manual says about removing the stator cover to check while the engine is running, "some oil may spray from the engine at this point" and that is an understatement. After a quick clean up, I check the strobe light and find it is back to not working again. I carefully peel back the still warm molten insulation of the outer layer to find a very fine coaxial wire which has melty issues. After a bit of very careful segregation I manage to squeeze in some insulating tape and do a re-test which is successful! So I checked the timing and it was spot on. The new float bowl gasket and increasing the float height by half a mm seem to have resolved all the leak issues and the bike starts and idles but still has the big flat spot off idle. In the meantime, someone has hit buy now on my auction and are now the proud owners of a 51yr old motorcycle!

In other words a good day in the shed!

[Laava]

Just the right buyer required.

Tribulations though. So, the advance mechanism works when revved?

Some ether spray when revved?
To be honest i didn't check to see if it was advancing. Dim of me. I have done the ether thing tho.