Topic: cb125j (or 125s 1976+) carb

[reinax]

Hi,

I have a cb125j (as it is know in the uk. i think it's just the s model from 76' onwards with the disc brake) with an old carb. I cleaned the carb with compressed air etc. but no matter what i do with the fuel mixture and the throttle screw, it keeps running so rich i have to replace the spark plug after every ride.

There seem to be two things i can do:
- replace the jets and screws. (I just made the order)
- buy a new carb.

About that second option: i seem to have a rather rare carb on the bike. I try to add it in attachments. As you can see it's pushed into the tubes on both sides, and not screwed onto the engine block. The picture of the carb in the brown box however, is the one i read about everywhere (including in my cb125j manual). Can anyone explain what's the deal here ?

But my real question regarding replacing the carb is: do i have to look for the rare carb that's similar to the one i have or can i replace it with other kinds of carbs like the i mentioned (i would have to able to screw it on the engine though one way or another...). Maybe it might be possible to make it real easy and just buy a new (replica) carb that's completely clean and functioning ?

Any suggestions ?

Thanks

[grcamna2]

reinax,
  Those carbs. are fairly easy to find Original used,or Chinese replica on Ebay;if it was me though..,I'd do my best to rebuild the Original one you have there.I'd use Original factory parts to do the rebuild if you can find them..,especially gaskets,etc.

[Tugboat]

+1

I had the same problem on my CB100 - come to find out I had two problems.. (1) one of the floats was bad, and was sinking; and (2) some PO had drilled-out the slow jet for some reason. Got a new float (was the same as CB350T's) and a new slow jet and that fixed it. The Keyster kits on ebay are okay, like the one below, but I'd stay away from any of the new re-pop carbs from Thai or whatever.. word is that they're of questionable quality.

http://www.ebay.com/itm/Honda-CB125S-CL125-73-75-Keyster-Carb-Kit-/161108645605?pt=Motorcycles_Parts_Accessories&hash=item2582d2d6e5&vxp=mtr

[reinax]

Thanks for the replies.

@ grcamna2 : easy to find ? I must be looking at the wrong places. After some worldwide researching i found 4 similar carbs as mine. Sometimes the carb was priced ridiculously, but most often they were sold to far away and the transportation costs were too high (i'm from Belgium btw).

As for the replica's ? i'f never found a model similar to mine... If you could point one out to me, i'd appreciate it.

As suggested; in first instance i'm waiting for a repair kit to arrive to restore my own carb. But in case i can't get the carb to work as it should i'd like an alternative.


One alternative would be to retrofit a carburator flange intake on my engine ( as in the other CB125 models ). That would allow me to install a whole series of carbs that are screw-on ( unlike the one i have)...

[grcamna2]

reinax,
     Take a photo of your cylinder head/top end of the engine for us please so we can see the carb. flange etc.
Depending on your cylinder head.,you may have a time retro fitting a standard CB125S U.S. spec carb... It's worth a view to make sure 

[reinax]

I Hope my carb shows up in attachment. the model should be completely similar to the first photo i posted. It's difficult to see in the image but the flange is screwed to the engine vertically (and not horizontally like most other cb125 models). I'll include an exploded image of my engine model.

A solution would be to retrofit the flange from the 82 cb125. There the flange is screwed vertically to the engine (like mine) but the other end screws horizontally to the carb. What would allow to install many other carbs ranging from all cb125 years. I'll also attach an exploded image from this later 82 model.
This however leads to the question of choosing the right carb...

The simplest method still remains to find an identical carb to the one i have and hope it's in a better condition then mine.... But the ones i found on the web looked just as crappy as mine...

[grcamna2]

I can see it. I'd love to have another 82'..,I really miss it w/ it's CDi ignition.

[reinax]

Here I am again with my engine problems. Six months later and the bike still isn't running great.
I did the carb rebuilt, but it still runs rich.

Who has some experience with tuning these carbs ?

I have a screw on the side that lowers the slide of the throttle and I have a screw on the lower front of my carb.

Someone gave me the following advice:
the one in the front of the bike, he claims, is the fuel mixture screw. To set this mixture screw, what you do is turn the idle screw (the one on the side) so that the idle raises to about 2000rpm.  Then you turn the mixture screw, and you will see the RPMs rise and fall a bit.  The screw is set right when the RPMs are at their highest.  If you notice that things are getting better when you get to the end of how much you can turn the screw, something is wrong and it'll be time for a rebuild of the carb.  Most likely this will work fine if everything is clean and now that your float is set right.

When you get the highest RPMs with the mixture screw, lower the idle back to normal, about 1200rpm and you are done. 

... right, this didn't work... When I turned the so called 'fuel mixture screw' all the way in, my rpm started going up. That's not right. If it would have been a fuel mixture screw, turning in the screw would limit the amount of gas going to the engine and ultimately stalling the bike.
 
Another reliable source claimed that this wasn't a fuel mixture screw nor a air mixture screw and that I just had to get the setting right by listening to the engine.

I don't think I have enough experience to tune the carb by listening.

Who has some sound, practical advice on tuning these carbs ( Keihin PD 66AA ) ?

Thanks

[grcamna2]

I think that if that is the correct stock carb. for you bike you should get the book and adjust the air screw(if that's what it is)as far out as you can to lean it out(someone may have screwed it in too hard & opened up the passage too much)and hope that's enough.What size pilot jet does it have in it ?

[reinax]

I've never found any information on this carb (keihin PD 66AA), let alone a book. I do have the haynes manual of my bike but there's another carb in there. I don't think that means my carb is not stock. I'm thinking the haynes manual might have a slightly different model (different country, different year of production). I've seen several pictures online of similar models that carry the same carb.
Just to say, I have no info on the carb and I wouldn't know where to get it.

I believe the size of the pilot jet is #42.

[grcamna2]

reinax,
I think that your pilot jet is too big,go back down to a 35 which is what it was stock if my memory serves me correctly...

[reinax]

OK, I just ordered a new pilot jet. I almost feel ashamed for not having tried this before .

It will arrive in 2-3 weeks.

If this works, i'll buy you a beer.

[grcamna2]

I enjoy Becks,but anything will do 

[jack_ellis]

Hello - I have a CB125J with (i believe) the same carb on with the flange system too.

I'm also having trouble with an over rich fuel supply even when i turn out the screw all the way

I thought i'd chime in with what I know:

(assuming I have the same carb as you)

There are 2 screws on the side of the carb. One is the idle speed (a knurled slotted screw) and the other is the air/fuel mix screw - brass slotted head.

Turning the air/fuel mix screw in cuts of the supply of air (not fuel) to the carb. Therefore turning it outward (anticlockwise) its adds more air into the mixture (lean).

I also Have the Haynes Manual for the CB125J which has a specific section for the details on the 125J. I will list:


_____________________________
Carburettor Settings:

Main Jet - 110
Slow running Jet - 45
Needle position - 2nd notch
Pilot Screw - 1 1/2 turns out
float level - 24mm
idle speed - 1,300rpm
_____________________________




I can't get mine to idle anywhere near 1300rpm so i must have this wrong too. Potentially the float height.

[grcamna2]

I would also check the float/float needle valve to be sure they are still good;I generally replace the float needle valve because over years the rubber tip has problems w/ the Ethanol fuel mix they use here.I suggest also fully cleaning out the fuel tank,even small microns of old rust can cause the float needle to overflow and flood the engine at different times.The little spring peg on the float needle valve also needs to move freely and where it makes contact w/ the float needs to be nice and smooth w/o a little dimple on that place on the float,it can be smoothed off on some floats.

[zark]

sorry to bump this up ....


But did you ever solve your issue . 

I have the EXACT same problem. In fact it's good that i saw your bike is blue, else i would think you're the guy who sold me the bike ... :p

ps: the PD66A is a special edition made just for the '78 cb125s ... which is why it's very hard to find. It was supposed to be better... but all i find is people on the net complaining of over-rich mixture ...

[grcamna2]

The PD66 carb. also came stock on the 1979' model/yr. CB125S,yes they Are tough to find a Good used one !
The key to finding a good used one is to check and be sure someone hasn't over-tightened(this is a fuel screw not an air screw so when you open it that richens it while closing it down some leans it) the mixture screw which opens up the passage too much.You can use any of the other model/yr. CB125S carbs and change the manifold to make it fit.