Topic: Honda cb125s idles but wont rev

[RUSTNBONES]

Hello
I got my hands on an old cb125s from  1985. Just the engine,so into an old dirt bike frame it went with new ignition coil,points,condensor,spark plug and pz26 carb, the valve clearances are done; timing and points gap done: have checked cam in its correct place at tdc: advancer counterweights all free
This old bike is on the 6V system and currently no battery ; just direct from engine to condensor ect,I get a good heqlthy spark and at idle its putting out just over 8Volts
But no matter what the carb or the jet size or needle position,the result is always the same, start and idle no problem,but as Itry to increase the revs,the bike cuts out:
I must have been through every setting and jet size possible:from 88 to 110
The result was exactly the same when i tried a pz 27 Carb on it,
AsI said before I only had the engine:so for the moment it has a pod foam filter and I managed to get the front section of pipe from an old xl125.Not ideal but enough to get runnung right? the aim is to get it running good and build a small cafe racer,
any helpful suggestions really apreciated:

[RAFster122s]

Have you put a timing light on the points to see if you are getting advance or if it is already advanced…
Lack of advance as it revs would make them conk out.

Also, are you certain the brass jets in the carb are clean, often if the jets have any deposits then the jets can’t work and it falls on its face.

[Alan F.]

For jets, 38 and 95 or 105 is what Partzilla.com shows.

[grcamna2]

The 1985 CB125S engine doesn't have points but CDi ignition.

[Alan F.]

So Bill do you suggest ignition troubleshooting instead of chasing carb issues? I'm only familiar with points and CB/CM400 cdi.

RUSTNBONES are you working from a manual?

[grcamna2]

So Bill do you suggest ignition troubleshooting instead of chasing carb issues? I'm only familiar with points and CB/CM400 cdi.

RUSTNBONES are you working from a manual?

Clarification about what the OP actually has/is working with..

[Alan F.]

I guess it answers David's question as well then.

[RAFster122s]

If you haven’t ultrasonically cleaned the carb I would start there, the small passageways in the carb can bet clogged with deposits… rereading your post I see you tried a bunch of carb jets but no mention of having cleaned the carb body or how it was cleaned.
Is the PW27 the correct carb for that motor?

Need more input to provide general troubleshooting tips…I own smaller bikes and bigger bikes, no direct experience with the 100-125 size other than having ridden a 100 long long ago once…