I have a maintenance question for my Honda motorcycle. I believe it is a CB125F in most of the world. I am new to doing my own work and I am trying to identify a hose (pictures attached).

Can you tell me what this hose is, and - most importantly - is it supposed to connect somewhere at the top?

What I have is a Chinese Honda "CBF125", which is not a cbf125 in other countries.

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It's a great, simple carbureted bike that seems to be

  • very close to a CB125E/F

for most places.

What I am stuck on is the hose in the next two pictures:

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I was putting things back together for the first time and saw it. It runs down to this round box and we see the connection in the next 2 pictures.

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Here's one last shot of the round box from front to back.

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  • Does the gas tank have a on/off/reserve switch on it? That looks like it's about in the right location to connect to it. Often, carbureted motorcycles have a vacuum operated fuel valve which will open when vacuum is applied and allow fuel to flow. That way when the motorcycle isn't running, gas won't be able to flow through the carburetors and flood the engine. The round canister might be a vacuum reservoir, which will hold vacuum whenever the engine is running. – the_storyteller Apr 18 '19 at 15:26
  • Looks like a vent to me. Battery vent? – David Watson Apr 18 '19 at 15:54
  • @DavidWatson, I don't think it could be a battery overflow tube because it goes into a vacuum box, and because the battery is below and behind it. – the_storyteller Apr 18 '19 at 16:02
  • @the_storyteller That's a fair assessment. It just doesn't look like a fuel line to me. Then again, I don't see anything else resembling a fuel line either lol. – David Watson Apr 18 '19 at 16:07
  • @DavidWatson - not a fuel line. A vacuum line which runs to a fuel control valve. – the_storyteller Apr 18 '19 at 17:31

It appears to be a breather tube. If so, it shouldn't attach to anything on the top side. It would be set up high so you could possibly ride your bike through deeper water and not get water into your crankcase while doing so.

  • I don't think this is a crank case breather. It's too small, and the crank case breathers on Honda bikes I've worked on runs from the crank into the intake before the carburetors. Also, we can see a suspiciously crank-case-breather-ish hose in one of the images, in the correct spot. Also, it doesn't connect to the crank case... – the_storyteller Apr 18 '19 at 17:36
  • @the_storyteller - You may be correct about the crankcase part of it, however, it still looks like a breather hose, which wouldn't be attached to anything on the top side. Also, there's no clamp or even clamp marks at the top of the hose, which leads me to believe it's never been attached to anything, lending credence to the breather theory. – Pᴀᴜʟsᴛᴇʀ2 Apr 18 '19 at 17:39
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    @Paulster2 - I thought some kind of vent hose as well, since it looks like the inside of the hose has the same color dust as the outside. – David Watson Apr 18 '19 at 18:02
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    @MikeM - I should update my answer a little ... the crankcase is the part which holds the oil ... where the crankshaft is at. After looking at the images again and what David Watson stated, what you're showing is not the crankcase, but I still believe the tube is some sort of breather. – Pᴀᴜʟsᴛᴇʀ2 Apr 19 '19 at 0:09
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    Have you checked that the tube doesn't connect to a valve on the bottom of the gas tank? If there is no valve, then you should just put the gas tank mostly in place, start the bike, and see if the tube has vacuum or pressure in it. If it has vacuum, then it probably needs to be connected to something. If it has pressure, or no pressure difference, then it is probably okay being left open. – the_storyteller Apr 19 '19 at 16:37

It's almost certainly your fuel tank evap / "breather" line

The round box is your "evap can" -- vapor management can that traps and releases polluting vapors through absorption and evaporation.
The paired question here directly addresses identifying the can.

A fuel tank vent or "breather" line is essential to this. Holding fuel vapor from the tank is maybe the biggest goal of this system.
If all the other hoses from the can are connected and none to the fuel tank, then we should definitely be looking for a fuel tank hose, and that one is going right up there.

  • Look for a matching connection on the back or bottom of the fuel tank and that's almost certainly where it's going.

(of course, make your own decision at your own risk, etc)

It may be something like this:

enter image description here

  • I welcome feedback since I'm new at all this. If you disagree, please share some more ideas :-) – Mike M Apr 20 '19 at 2:23

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