Topic: What is this thing?

[noisebomb]

I am not sure what this part is.  It was attached to the bottom of the battery box.  A hose came from the bottom and went out the rear of the bike; it was capped off at the end.  Another hose came out of the 'angled' outlet, it led to the valve cover.  A third hose came out of the top outlet, it was connected to nothing. 

I was able to remove three of the four screws (Why is it, when you have four screws; the first three come out no problem, but the fourth strips?).  I carefully twisted it open, it was empty.  It also smelled of gasoline. 

If you replace the hose from the valve cover with a filter, do you still need this thing?

Thanks!

[robe0531]

it is for emmisions on a 77/78 cb750 not sure about other years

[Alan F.]

Yeah, I think Rob's nailed it on he years.  I think it's easy to get rid of, it functions kinda like a PCV valve.

[robe0531]

i dont run mine one of the first things i ever took off my bike

[Steve F]

I left mine on...doesn't hurt anything, but if you're running pods, there's no place to hook everything up to.

[eurban]

Its not a PCV valve, it serves as a fluid separator.   The stock setup has the crankcase fumes routed back to the airbox but if you connect the hose directly from the back of the head to the air box you would get nasty crankcase spooge contaminating your air box/filter.  The separator collects the liquid before the gases flow into the air box.  It is supposed to be drained every once and a while. . . .I have found that if one uses a crankcase breather filter directly on the back of the head(for instance when pod filters have been installed and it is no longer possible to route the fumes back to the air box) that the fliter will get very "greasy" rather quickly, severely limiting its ability to release crankcase pressure.  With a big bore kit in particular, it is critical to have good air flow potential to release this pressure, otherwise oil leaks / seal failures can occur.  A greasy filter is not up to the task.   I have actually installed an after market separator with 3 hose inputs (one from the stock location on the head and two from hose barbs installed on the tappet adjuster covers) and with a large capacity filter to release pressure.  This definitely helped with oil seepage on my 836 project.  Basically I think that a separator is a good idea even if you have ditched the stock air box.  BTW, the fumes from the crankcase are pretty nasty stuff, and routing them back into the airbox helps to keep them out of your lungs and clothes.  Keep the stock setup if you care about such things!

[noisebomb]

Thanks for that.  Good to know.  

Is it supposed to have anything in it?  Like a filter or something?

Also, I know where two of the hoses route too, but the third was just lying on the engine.  I have no idea where it goes.

Thanks for all the info!

[Alan F.]

Its not a PCV valve, it serves as a fluid separator.   The stock setup has the crankcase fumes routed back to the airbox but if you connect the hose directly from the back of the head to the air box you would get nasty crankcase spooge contaminating your air box/filter.  The separator collects the liquid before the gases flow into the air box.  It is supposed to be drained every once and a while. . . .I have found that if one uses a crankcase breather filter directly on the back of the head(for instance when pod filters have been installed and it is no longer possible to route the fumes back to the air box) that the fliter will get very "greasy" rather quickly, severely limiting its ability to release crankcase pressure.  With a big bore kit in particular, it is critical to have good air flow potential to release this pressure, otherwise oil leaks / seal failures can occur.  A greasy filter is not up to the task.   I have actually installed an after market separator with 3 hose inputs (one from the stock location on the head and two from hose barbs installed on the tappet adjuster covers) and with a large capacity filter to release pressure.  This definitely helped with oil seepage on my 836 project.  Basically I think that a separator is a good idea even if you have ditched the stock air box.  BTW, the fumes from the crankcase are pretty nasty stuff, and routing them back into the airbox helps to keep them out of your lungs and clothes.  Keep the stock setup if you care about such things!

Thanks, So maybe I should opt for one on my K3 build?  Could you link us to the type of seperator you used?
-A

[jumpjg]

Thanks for that.  Good to know.  

Is it supposed to have anything in it?  Like a filter or something?

Also, I know where two of the hoses route too, but the third was just lying on the engine.  I have no idea where it goes.

Thanks for all the info!

I think it is pretty much empty. There is a cone-shaped plastic frame inside & that's about it.
One hose comes from the valve cover, one hose goes overboard, and one connects to the hose barb on the back of the crankcase.

Joe in St Louis

[noisebomb]

one connects to the hose barb on the back of the crankcase

I thought that was connected to the oil tank.

hmmm....what about the top of the Airbox?  Does something attach to that?

Maybe it will show it in the manual...

[noisebomb]

Found it!  The hose I was unsure about goes to the top of the airbox.  It seemed like that would be right, the hose was the perfect length.

So, the hose from the bottom of the oil tank goes to the connection on the back of the engine, right?