cb125J carb jetting

[ZARk]

I'm restoring a cb125J.

But am having some serious issues with jetting.
The engine is running so rich it's killing spark plugs.

What has already been done :

* Cleaned tank
* Changed fuel filters
* Cleaned carb (ultrasound bath) + rebuilt with official keyster kit (KH-0160)
* Changed gasket between engine flange and carb
* Checked timings
* Changed HT coil + condensor.
* Removed airbox for a cone
* Changed exhaust for slightly shorter "megaphone" type . (with silencer and fiberglass sheet, so that shouldn't change much).

Symptoms :

The engine runs, i can keep it idling with idle screw set almost at max.

I tryed moving the air/fuel screw to leaner, but it doesn't seem to make a differnence (whereas in plenty of videos you clearly hear rpm getting higher/lower).


I'm starting to think my carb is somehow broken, but i am finding very little documentation on this PD66A keihin carb.... (+ the engine mount is kinda particular).


I've ordered a colortune , hoping that might help a little, waiting for that badboy to arrive ...


Any one with experience on a S3 / J or N with this carb ? I'm pulling hairs at the moment ... And i'm seeing a couple of cry for helps with similar issues on the net, but no answers ...
Any advice would be good, even help in finding a compatible mikuni carb to fit.

 

[jpmobius]

Very likely the only problem is that your carb is not clean. Very commonly I will get a bike that runs badly or not at all and the report is "It's not the carb 'cause I just had it ultrasonically cleaned, professionally rebuilt, has all new rebuild kit, etc etc.". If your pilot air or fuel screw has no effect than 99% chance the pilot circuit is dirty. Take your carb 100% apart and manually clean every passageway with spray carb cleaner. It is much easier to do if you take the trouble to learn how each passageway works and whether it flows air, fuel or both. Plenty of youtube and articles on the interweb. Find all the brass parts you removed and put them back in and throw away all the parts except gaskets that came in the rebuild kit. If any of the existing brass is not OEM throw that away as well and buy OEM parts. Very likely your bike will be instantly fine after that. This is a very common experience in my shop, and usually I don't get the original carb parts with the bike but am able to find the correct OEM brass in old used carbs. Otherwise I buy new OEM. The brass jets and needles wear very slowly or not at all and even really old OEM parts were made very precisely compared to aftermarket and are generally far superior. At the least they are in fact the correct parts. Aftermarket jets are notorious for deviating substantially from OEM parts and can make you crazy trying to tune an altered bike. If your bike is stock, put it all back together with stock parts and it will run great.

 

[ZARk]

Thank you for your answer jpmobius.


I will do what you have suggested, but i should add a couple of details with this carb... I have odd things with the mixture screw.
When i got the bike, the screw that was in there was missing the o-ring and spacer, and the screw itself had the needle end slightly bent ...
I replaced them with the rebuild kit off course. But i'm concerned about how that needle got bent, and if it damaged the circuit it's screwing into ...

+ when opening the carb again last night, i noticed the pilot jet is a 40 instead of the factory 45.