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.
I am very interested in this one part that seems to be the center of a system I'm not familiar with (pictures attached).

Can you tell me what this round black box is?

I can see 3 hose connections to other parts and one out to the ground.

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

enter image description here

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

  • very close to a CB125E/F

for most places.

This interesting part fits in your hand. It is cylindrical with the axis running horizontally from the center to the right side of the bike.

Here is the location at the right side of the bike:

enter image description here

Here is the face of it at the side of the bike:

enter image description here

Here is a shot of the whole thing from the back:

enter image description here

It's not easy to see from the other side of the bike, but if we get close we can see another large hose connecting from above:

enter image description here

And if we go back and look at the bottom of it, there is one hose down to the ground:

enter image description here enter image description here

What is this thing up to? :-)

  • I believe that the hose at the bottom is to exhaust the fuel in the system if the bike falls on its side. Apr 19, 2019 at 14:25
  • from what I'm learning, I think that's part of it
    – Mike M
    Apr 20, 2019 at 1:17
  • I've got a pretty good idea about this now, after extensive internet-ting. So I'll write an answer, but I won't mark it Accepted right away, in hopes of some feedback.
    – Mike M
    Apr 20, 2019 at 1:37

1 Answer 1


It's your "evap can"

  • fuel-evaporation-&-other-vapor management canister.

It's a simple emissions control system on many bikes today.
The idea is to trap polluting vapors from the fuel and air systems (other than those ejected in the combustion exhaust, of course).
This includes vapors arising from evaporation in the gas tank as well as some from "breather" systems that vent the crankcase and possibly other things.

The box is usually full of something like activated charcoal that absorbs the hydrocarbon vapors when they develop - such as lots of evaporation in the fuel tank on a hot summer day.
Then as the engine runs, it's air intake pulls air through the canister and draws the stored vapors out of the charcoal and into the combustion process.

Here's some discussion about this elsewhere online (better links welcome):
* https://www.bikesmedia.in/reviews/motorcycle-evap-system-explained.html
* https://www.quora.com/How-does-EVAP-system-work-in-bikes

Here is a link to a parts-store diagram of a similar canister on a different Honda (this SDH125 is not easy to find in the English-language internet):
* https://www.partzilla.com/catalog/honda/motorcycle/2007/xr650l-ac/evap-canister-ac

It has 4 probably very similar tube connections.
The two on the face of yours are at the top of the canister in that diagram.
Then on the other side are the same larger one going up, and the smaller one going down and out.

I believe they function as follows (feedback welcome):

  • The larger one on the front face is the connection to the engine air system - receiving excess vapors when the engine is less active and giving up the stored vapors when the engine pulls enough
  • The smaller one on the front face connects to the fuel tank vent - collecting its evaporative vapor when the engine is less active, and then allowing air to pass back up as fuel is drawn down when the engine is running
  • The large hose at the back is not going to another part but connects to outside air - it is on a valve that is closed at rest and allows inward flow when the engine air intake pulls, creating the return of stored vapors
  • The small hose to the ground is an overflow - if there was so much vapor that liquid developed and exceeded the capacity of the charcoal, the liquid could exit there
  • Does it have a life span? I mean does it require regular changing? Apr 20, 2019 at 10:30
  • so far, what I've read is that it's intended to last the life of the bike; it just keeps exchanging. But it looks pretty easy to replace if you wanted to.
    – Mike M
    Apr 20, 2019 at 11:52
  • there are similar systems that also process oil that I'm reading could be cleaned and emptied. I am new to all this, though, so I'm definitely not an expert :)
    – Mike M
    Apr 20, 2019 at 11:56

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