CB125 Wont start - it did but would not idle?

[southpoint]

Hi Everyone,
I am just starting on my first project - just passed my CBT and got myself a CB125 C Superdream. When I got it it was running and ticking over real rough so I thought a Carb strip down is in order:

So I stripped down both carbs and meticulously cleaned the carbs, blew out all the jets and eveything with compressed air, set the idle mixtures to 1.25 turns as in the manual, set the idle stops on both carbs to be the same and went to re-assemble.

I reassembled and set everything, made sure the chokes were working bang on synch - did a finger test for roughly setting the slider heights prior to a vacuum test and both sliders were moving well and picking up at the same height etc..

I thought GREAT - let's get some fuel in and give it a go.

First couple of turns took a a while to get going but then it sprung into life on full choke - It worked OK on the throttleand revved but as soon as it went to idle it died quick.

Turned up the idle stop a quarter turn started it and dropped it to idle and it idled for a very short while then just gradually died [ revs just gradually getting less] - not quick just a slow gradual stop as if it was gradually getting starved or something was slipping.

The weird thing is now -I have not changed anything and it will not start at all not even on the throttle - just keeps turning over but will not fire - choke or no choke?

Can anyone give me a clue as to what may be the problem and what to look for - like I said it is my 1st bike and is a a big learn for me.

A few pointers would be great as what to look for and check.

Cheers.

[koolio]

Potentially could be many things, more info needed.

Did you actually remove the idle mixture screws when you cleaned the carb? You will need to do this and replace any o-rings there.

Are you running the carbs with the air filter on?

Did you check float height?

Try this:

Open the idle mixture screws fully (anti clockwise or clockwise depending on your carb), if you over open them on CV carbs for example, they also dont work so dont over do it.

Then try starting the bike without the choke if you can, you can give it some pull on the throttle while starting (but make sure you don't over do it).

When the bike starts keep it on lower revs holding the throttle slightly open, when sufficiently warm release the throttle, if the bike dies you know you have an idle system problem.

Remove the carbs and go through them again thoroughly.

This manual helps greatly for understanding:

https://wwwclassiccyclesorg.mediafire.com/?c1wk828k2wtri5i

[nightshaddow]

the problem with these twins is there is 2 of everything,

2 cylinders 4 valves 2 carbs etc etc i find with mine it hates being out of balance, mine is the later tde model but is essentially the same bike.

first thing i would do with this bike is a full service, oil, plugs, valve clearances etc and as i am sure teflon mike will tell you make sure you clean the oil strainer found inside the clutch cover.

once serviced if you have one available i would balance the carbs with a carb balancer. if you dont have one i would buy one as for a garage to balance them for you would cost an hours labour which is between £35 and £50 for most garages, my balencer cost me £25

https://www.ebay.co.uk/itm/Motorcycle-Carb-Carburettor-Vacuum-Balancer-Gauge-2-Cylinder-Gauges-Balancing-/280881813602?pt=UK_Motorcycle_Parts&hash=item4165dc5862

i had helish problems with mine due to scrotes changing carb slides and all sorts so i would have a good look at all the parts and make sure it all matches up
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"WHAT DO YOU MEAN 100 MPH THATS IMPOSSIBLE Its chinese OFFICER."
Current rides CB 125 TDE(the next project) NS125RK ( now on the road ) CB125TDC (soon to be project 17) melody ( the nephews )

[Usher4566]

Not all twins have two of everything my twin has single points single coil single carb
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New cd200 benly owner

[nightshaddow]

[Robby]

By the sound of it this isn't on the road - just started up and sat ticking over, gradually running weaker, then dieing after a couple of minutes?

Do you have the fuel turned on?

[southpoint]

Thanks for replying everyone,

I stripped it down again and went through it just as I did before, reassembled it and then noticed that the fuel flow from the tank was slow- checked the petcock and spotted a load of crap had gone in there from the tank and almost blocked it, I did clean it before but obviously not good enough.
Cleaned it out swilled a gallon of petrol through the tank and put some fresh in.

Started up straight away - dammn crap in the tank had done it - ahh well I will know now to check that first.

Thanks again.

[Robby]

Fit an inline fuel filter - if there was enough crap in the tank to block the petcock, then more will make its way down to the carbs over time. The paper element filter tend to perform better than the mesh ones.

[southpoint]

Thanks Robby,

Do you have any recommendation as to a brand of filter and the best place to get it?

[southpoint]

Well back on with my project for a while yesterday.

Bike had not been started for a few days - came to start it and it would have it for a while, eventually fired but wass idling real rough.
I noticed that the left cylinder has getting a lot hotter than the right one and I presumed it was because the carbs were out of balance and the left sylinder was doing all the work? Anyway got out my nice new shiny Vaccuum guage kit to take a look. The dammn threads on the brass stems supplies with hte guages are too small, I measured them and they were 5mm - the holes on the bike are 6mm.
I see that I can get a 6mm adapter or indeed I spotted that someone had said you can use welding tips - does anyone have any advice?

There was something bugging me about that left carb and its idle mixture. We had stripped everything off on the carbs, cleaded blown out with air etc.. and reassembled. The mixture screws were set to 1.25 turns as said in the manual and all was happy days although the thing that kept bugging me was that when the left carb idle mixture screw seated down it was at a funny angle - the other screw had the slot horizontal to the edge of the float bowl this one was way round.
Anyway as a test I gradually would in the mixture screw on the left carb bit by bit, compensating for it by adjusting the idle stop a tiny amount just to stop it stalling. Well the upshot of the experiment was that wound in the mixture screw all the way totally closed and it did not make any difference apart from the idle being a bit lumpier - THAT CANT BE RIGHT CAN IT??? are the seals on the mixture screw shot do you reckon and it is just leaking fuel through?
When the bike was idling the thing I also noticed is that if I turned the idle mixture or the stop on the right carb it made hardly any difference at all - but the left carb made a big difference - which is why I thought that the left one was doing all the work.

I am confused now - I am going to strip the carbs back off again and re-clean and look at the mixture screw seals - Any ideas guys?

Cheers

[koolio]

Sounds like a carb issue to me (although could be timing). Really clean out the carbs and all the jets using a very fine guitar string, make sure all the passages are super clean. Replace idle o-rings that are obviously compressed or really should get replacement.

Study a carb manual (if you haven't already) so you will exactly know what you will need to clean out to where.

This is the one I meant to show you before, its a very good source.

https://dl.dropbox.com/u/106752/Manuals/Honda/Honda_Motorcycle_Carburetion_1975.pdf

Litmus test is swapping the carbs around and seeing how the bike runs then.

[southpoint]

Thanks Koolio - that manual is great but has made my earlier post make me look like a complete idiot LOL

I just wrongly assumed that the mixture screw was controlling fuel but it isnt is it it controls the air at idle when the sliders are down on the stops - makes far more sense now. I suppose if the seal is broken and not seating properly the left cylinder is letting too much air through which in turn draws more fuel out - hence it was running more powerfully than the right one - I am on track now or have I got it wrong again?

I suppose the way to go is to replace the idle mixture screws, seals and washer and we should be back in business again after another good clean out and reset.

Just out of curiosity on old carbs like these do you normally have to make any adjustments to the mixture screws once the carbs are back on to get some initial balance prior to vacuum balancing - tweaks - or is it normally all done via the throttle stops and the slider top screws when balancing with the vacuum guages?

[nightshaddow]

it might be worth doing a compression test on both cylinders, no matter how much you fiddle or adjust you will never get the carbs balanced if there is something wrong with the engine,

https://fbcdn-sphotos-a.akamaihd.net/hphotos-ak-snc7/418283_10150671171848846_987263031_n.jpg

these are the pistons out of mine as you see one is badly scored the other is fine. i also needed to re grind my valves as they where pitted and i had similar symptoms as you

first thing i would do is whip the rocker cover off and set the valve gaps,

next fit brand new plugs and gap them correctly, set both mixture screws the same and try to balance the carbs with the carb balancer a bit of ptfe tape will do the job till the new inserts come.

if you cant balance the carbs once you have done all this you need to look into your engine for the problem
____________________
"WHAT DO YOU MEAN 100 MPH THATS IMPOSSIBLE Its chinese OFFICER."
Current rides CB 125 TDE(the next project) NS125RK ( now on the road ) CB125TDC (soon to be project 17) melody ( the nephews )

[southpoint]

Thanks mate - I see what you mean.

I will get the valve clearances done and strip the carbs again, re-clean them all and replace the O rings in the idle mixture.

Then give the balancing a go.

Will let you know how I get on.

Cheers

[blurredman]

Make sure there is no gaps in the manifolds. They can prevent a proper vaccum and can do similar to this too.

Possibly use the instagasket stuff and make a seal on the manifolds. This is 30 year old rubber/plastic.
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CBT: 12/06/10, Theory: 22/09/10, Module 1: 09/11/10, Module 2: 19/01/11
Past: 1991 Honda CG125BR-J, 1992 (1980) Honda XL125S, 1996 Kawasaki GPZ500S, 1979 MZ TS150.
Current: 1973 MZ ES250/2 - 18k, 1979 Suzuki TS185ER - 10k, 1981 Honda CX500B - 91k, 1987 MZ ETZ250 (295cc) - 40k, 1989 MZ ETZ251 - 51k.

[koolio]

[southpoint]

Hi Guys,
Latest update on the project and all seems to be going well.

I followed the great advice you guys gave me, sorted out the carbs and the vacuum and bike is running real nice - good tickover at 1300 and nice power when it is revved - happy days!

I have however come up against a new problem now - when the bike is idling in neutral it is great I can flick it down into first no problem, revs dont dip, although the gear change is a bit stiff I can then flick it back into neutral BUT if I try to get any other gears it is almost impossible and really stiff on the changer - the bike is standing and not been road tested - if I do it by hand with a really sharp tug it will go though from Neutral to First and also though the other gears up and down - although sometimes it is a struggle - but if I try to change it when im on the bike or when its moving slowly it doesnt seem to change at all and cant get back to neutral and I have a problem gettin through the gears.

Can anyone give me any pointers where to look.

Cheers.

[Usher4566]

Mine was doing this when clutch cable was adjusted wrong .in the end I changed clutch plates and springs because couldn't get it right .plates was worn past limit and springs shrunk
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New cd200 benly owner

[Usher4566]

Well done on geting it running better .how much freeplay in clutch ?
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New cd200 benly owner

[southpoint]

Hi Usher4566 - thanks for getting back to me.

I have had a chance to look at the freeplay today and the gap on the lever between the end of the lever and the post is 11mm so I suppose that is OK? - says between 10 and 20 in the book.

So I had a thought about doing a few quick checks on this:

When the bike is stood still, engine off, hot or cold I am guessing that like a car I should be able to go throught the gears one by one and get back to neutral pretty easy - it dont work - I cannot get through anything!

Occasionally I can get 1st and then back to neutral but to get through the gears it takes a real sharp jab on the gear pedal by hand and sometimes I cannot get through the gears at all even at standstill - no gears and getting neutral is difficult.

I am thinking that this may be due to the bike not being run - maybe the gears will change and settle when it is actually moving? I have just rebuilt it and have not road tested it yet - just checking where it is stood - but not touched the gearbox or clutch?

If I have the engine on and I select 1st the clutch seems to bite OK although it seems loose but the bike pulls away OK.

I am confused as to what to look for - do I need a new clutch assembly do you think?

[Usher4566]

I don't think mine selects gears unless moving .you need tension on chain .you need to ride .but if you do need new plates and springs its so easy to do took me about 40 mins if that .and fixed my problems strait away
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New cd200 benly owner

[nfo]

Isn't this just due to bike gearboxes having no synchromesh? If the dogs are meeting end-on with both the main and counter shafts stationary (i.e. the rear wheel isn't turning, the clutch has been disengaged long enough for the input shaft to come to a stop), then they'll never engage. You need some relative rotary motion between the sliding gear and the gear it locks to the shaft.

Here's a photo of some Honda gearbox innards:
https://i278.photobucket.com/albums/kk83/no_bb/CB125-SML/DSCF6871.jpg

(Exploded diagram here: https://images.cmsnl.com/img/partslists/honda-cb125s-s1-usa-transmission_bighu0213e4111_8d76.gif)

You can see that on the countershaft (which is at the top), the dogs on the third and fourth cogs from the left are meeting end on, whereas to engage successfully the dogs on one need to drop into the gaps in the other.

Correct me if I'm wrong, but with the bike stopped and engine running, if you start with the clutch engaged and the box in neutral, the mainshaft will be spinning. Pull the clutch in, engage first as it spins down and both shafts will now be stationary (you've connected the mainshaft to the stationary countershaft via 1st egear). If you're lucky, the dogs on second gear will be aligned correctly, and you'll be able to shift up, if not they'll clash.

If I'm right, rolling the bike back or forward a little should allow the dogs to engage. The honda gearbox has sliding gears, I'm more familiar with a sequential gearbox where the gears don't slide and there are separate dog-rings.

Hope that makes sense, I'm thinking on my feet.
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Mar's Law: Everything is linear if plotted log-log with a fat magic marker.
1975 CB125S 'repair and tidy up' project thread here...