Honda cb125s idles but wont rev

[siMcfixit]

So back here again after revisiting the dirt bike snoozing in the shed.i had a frame and cb125s motor,so together they went.
The carb,coil condenser and points are all new. Spark is good, at the moment I am running that from the black wire from engine.(puts out 8.5v)
These old girls are 6v systems .
The bike will fire up and idle, but will not rev.in some cases you can feather the throttle a bit. But as soon as you get(on the needle). Bike cuts out. The points gap,timing, valve clearance all done and revisited many times. As with the two centrifugal weights behind the timing cover. So all that stuffs good. It was a pz26 carb that came off.and a new pz26 that went on. It does not make very much difference which main jet I use#88 up to #110. Still exactly the same.problem.and plug new too. Caramel in colour. Is this an airbox and exhaust thing. I did manage to find an old so 125 pipe to get things started.and at the moment is a foam filter on carb. Not my choice, but what's available.any one any ideas?
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You will always meet nice people on a Honda.

[A100man]

Decent compression?
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Now: A100, GT250A, XJ598, FZ750

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600

[WD Forte]

Insufficient fuel flow to keep up with demand?
check usual suspects for this
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bikers smell of wee

[Mysteriass]

Was about to go to work one morning and something like this happened to me.

It was still dark at about 5am. Ironically, the darkness was what allowed me to see that one of the ignition wires was shorting onto the frame. The sun had gooified the electrical tape holding it all together and sloughed off. It looked like a disco down there by the coil. Don't ask me how it started and ran like that but it did. Just wouldn't rev.

I did an image search for the model of bike to show an example of it. I can't believe it but the first page of images turned this up:

https://i.pinimg.com/originals/50/73/c7/5073c7c84d30c0ce4e718f3e7a2e6955.jpg

I'm certain this is actually my old bike or what's left of it. Last I heard - many years ago now - the lad who bought it popped a wheelie (Somehow) and the front end dropped out of it. Pity really because it was a handy bit of kit. It'd do about 40mph on a misty morning and loved it. Maybe someone fixed it up and is still riding it around.

[Robby]

Assuming your sparks are ok, the problem would be fuelling. It sounds like a frankenstein bike so broad-brush stuff.

It runs at tickover so you probably don't have a gigantic air leak and fuel is getting through. It dies when you open the throttle, so it's either getting way too much fuel or hardly any fuel. I'm going assume the problem isn't just that it needs some choke.

You mentioned a new carb. Is that a decent new carb, a good used carb, or a cheap new ebay copy?

Things I would check:
1. Fuel. Is it fuel, or is there a load of water in it?
2. Needle. Is it connected to anything, or just sitting in the top of the jet.
3. Main jet/emulsion tube. Have they dropped down into the float bowl. If they are in place, can anything get through them?

[siMcfixit]

Hi
Thankyou for your replys: yes a frankenstien bike,I have tried several times to upload a picture,
The compression is good: fuel fresh and what little wiring there is, fresh cable and not arcing on frame,
its the correct size carb on the motor and says its a kehin,but given I paid around 30 for it from the amazon people ,my doubts started there,Ihave gone round it with the calipers to check all the diameters are the same,
fuel deffo gets through,and depending on which jet gets to spend ten minutes in there, will vary if the plug comes out wet or dry,
I am still not convinced about the setup with the foam filter or lack of silencer, i did find alot of comments on other sites about how theses motors dont like running without the proper airbox and exhaust,sometimes old things dont like change, onwards.
____________________
You will always meet nice people on a Honda.

[A100man]

What happens if you take the air filter off completely?
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Now: A100, GT250A, XJ598, FZ750

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600

[siMcfixit]

Hi, next to nothing,no real noticeable change with or without that filter.i can get my hands on a honda nx125 frame with airbox intact.starting to think this maybee the way to go anyway.
In fact ,the more I think about the "air side" that filter is doing not very much,which explains why the most i can get out of that bike is when the choke is in the half closedposition thus restricting the airflow a bit. ?
____________________
You will always meet nice people on a Honda.

[Bhud]

Your carburettor likely has a blockage.

I don't think it's getting enough fuel. You should disassemble the carb and I think you may find a blob of rust particles or a chunk of dirt in there.

[siMcfixit]

Hi,thanks. I know the main and idle jets are good .and it's possible to lean out or riches the mix with the small fuel screw located down under next to the float chamber. So that doesn't leave much. Though no harm pulling it to bits and introducing it to a compressor..
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You will always meet nice people on a Honda.

[Robby]

The mixture screw only affects the mixture at idle and tiny throttle openings, it won't make a big enough change to help here.

The main jet is only important at full throttle and close to.

Everything else is on the needle. Make sure it is connected and moving. If it is, try moving it a notch higher.