Topic: difficult on setting idle on a cb125s

[mazingerzeca]

Hello all.
Well, it's being difficult getting the accurate idle on a Honda cb125s (J model, the one with a bigger tank). The engine has been rebuilt with new valves and piston.
The bike, made by Honda Italy, was equipped with a 22mm Dell'Orto, but it was scrap, so I decided to buy a Keihin thai replica on e-bay, which as far as I know, is the one which equipped this bike for the international market.
This one was also a 22mm, but it had the 100s jetting sets, which were 38 slow jet and 105 main jet. With this settings the bike worked, but it was impossible to maintain the idle withouth pulling the throtle, and it lost power on high revs, so I bought the 125 jets (45 slow jet and 110 main jet).
With these settings the bike is much more better, starts at first kick, but when you are at high revs and you leave the throtle suddenly, the needle goes down quickly and the bike stalls. For this not to happen, you have to set idle high, at 1600 rpm at least. Even though, it happens, and it stalls on traffic lights, if it lasts too much.
To add more, recently, I read in Classic Motorcycle Mechanics some articles about the rebuilt of a cb125s (j model), which was equipped with a keihin 24mm, bigger than mine.
So, these are my options:
- Buying a 24 mm carburetor (no idea of which settings to use on it)
- Given that the thing improved putting a bigger slow jet, should I use one bigger than 45?
I've tried everything, mixture enrichment, needle height, everything, what can I do?
Thank you.

[martino1972]

did you clean that carb.. there are some pretty small orfices in there,especially in the idle jet area,if not clean the idle jet can't do it's job properly and the engine dies.....

[mazingerzeca]

No, the carburetor is brand new, absolutely clean.

[mlinder]

Have you checked plug deposits for the idle circuit?

[mazingerzeca]

Yes, it is clean. In fact, the bike works almost normally. It is just that it doesn't keep the idle under 1500 rpm.

[1974blaze]

i wish i had an answer for you. i have just bought a 78 125s and after cleaning the crap and i do mean crap out of the carb it runs, sorta, but i can't get the idle to settle down and if i do i cant get it to run properly without keeping the rpm very high.  i'm sort of convinced that this is because the keihin carb i have in there (stock) does not have a removable idle jet.  so despite blowing cleaner and air through it i might not have it perfect.  does your idle jet unscrew or is it fixed?

[mazingerzeca]

My idle jet is removable. The carburetor I have installed is a fake thai Keihin 22. I have tried with a #45 jet; with this jet the air screw was turned 1 and a half turns out. I've tried also with a #38, which required just 1 turn out. On both cases, the idle has to be set at 1500. The fact is that it needs to be rich on low speeds for it not to stall, no matter which jet you use, so maybe it is due to another problem  apart from carburation.
This is the carburetor.

[Raul CB750K1]

Ignacio, I may very well be wrong, but according to the diagram, you need to set the idle with the cable screw.

Usually, the idle is set "mechanically", that is, there is a screw that actuates the throttle only slightly to set the idle, a screw that sets the throttle stop. In that carburetor, there is a screw above through wich the throttle cable goes to lift the slide. With the throttle in rest position, you have to unscrew the cable adjuster until it raises the slide and sets the idle, obviously after you have set up the mixture air screw and installed the correct jets.

It is very similar to the way you adjust the clutch cable.

[mazingerzeca]

Yes, Raúl, you could do that, and yes, there is that kind of screw on the top of the carburetor (Nr. 5 top), but that is only used to adjust the carburetor to de cable lenght. The idle, itself, is adjusted with screw Nr 7 right, which, if turned in, raises slide Nr 8, and if turned off, leaves it rest on the carburetor. Nr 7 left is the air screw.
The problem that I have now with the bike is either idle jet #45 or #38, it needs to run rich, otherwise, the bike stalls, and I have to set the idle to 1500. The plug, after running for a while, is coffee color, so it should be ok, but, what happens on this engine to need so high idle? bad tappet adjustment? float valve height? problems on ignition?

[crazypj]

Ignacio, I may very well be wrong, but according to the diagram, you need to set the idle with the cable screw.

Usually, the idle is set "mechanically", that is, there is a screw that actuates the throttle only slightly to set the idle, a screw that sets the throttle stop. In that carburetor, there is a screw above through wich the throttle cable goes to lift the slide. With the throttle in rest position, you have to unscrew the cable adjuster until it raises the slide and sets the idle, obviously after you have set up the mixture air screw and installed the correct jets.

It is very similar to the way you adjust the clutch cable.

 That's not the right way to adjust a throttle cable.
 You need some slack(2~3mm) so cable doesn't operate slide when you turn steering (check manual )
I cant believe your advising someone to do it.
There is a throttle stop screw on the e-bay Kei-Hin/Mikuni.
Its more likely the pilot screw setting needs messing with. (its an air screw if on airbox side, further out leans mixture, further in richens it)
There is also a possibility the drilling's in carb are not complete or may be blocked with something.
Just because its new assume nothing and clean it ( Chinese 'quality control' leaves a lot to be desired)
 Clean it first, then think about a larger carb ( it will 'wake it up a bit', if your prepared to rev it)

PJ

[Raul CB750K1]

Ooops, you both are right, I was talking by memory of my Sanglas Amal carb but then I remembered there was a screw in the carb body that served as a stop for the slide, in such a way that it is possible to raise or lower the rest position of the slide.

Please disregard what I said in my earlier post.... (ashamed)....

[Raul CB750K1]

Is the ignition timing properly set up? First, set up the point gap as specified. Second, adjust the points plate so the points open exactly when the mark is on the "F".

Furthermore, ¿what's the idle speed of your bike? Different bikes have different idle speed, it depends very much in the engine's flywheel. With a small flywheel it is not possible to achieve low idle speeds

I may be wrong again, that would be twice in the same post and day.... :-)

[crazypj]

Points 0.012"~0.016". set them in the middle (0.014")
 Check advance mechanism is working
 Tappets, same as all sohc two valve/cyl screw adjust Honda's, 1966~,
 0.002" inlet 0.003" ex.
 Idle speed, about 1200rpm, never seems to vary by more than +/-100 rpm whatever model your working with
 I'm sure its a design feature that some settings are standardised?

PJ
 Raul, we all have 'off' days, I've had off years 

[mazingerzeca]

This is the used plug:

[crazypj]

That looks pretty hot, did you check for air leaks on manifold or leaks on exhaust?
PJ