What are these tubes on my bike?

[alehman]

Hi, I've been reading this forum for a bit, and this is my first post. I'm absolutely new to motorcycles and just bought a beautiful 1979 CB750L. She needs a lot of work before being roadworthy but is very close to running.




The bike is mostly present, but I suspect various parts are missing. Let me preface the rest of this by saying that the diagrams that come in the manuals give me little to no idea not only of what is missing, but also of where parts go and how they're installed. I've figured most of the stuff out by browsing this forum and others, but I finally have some questions I'm stumped by.

There are two tubes that come up from the rear of the engine and connect to each other.



I also have this tube that seemed to be wedged among the other tubing, connected to nothing.




From what I've read, I have a suspicion about what these tubes are and how they're meant to go, but I wanted to know for certain from you guys. The small tube looks to be the right size to connect to the crankcase breather reservoir box underneath the battery. The drain line from that box is connected, so it would have to be the one leading from the engine if it does go there. I suspect that the two tubes that connect to each other were actually meant to connect to a T-piece that then connected to the smaller tube, which in turn fed the reservoir box.

Is this right??? I can't find any literature elsewhere that tells me how these tubes are supposed to go.

[Piute]

 Sure some-body know till then http://www.bikebandit.com/houseofmotorcycles/1979-honda-cb750k-10th-anniversary-edition/o/m143619#sch16973

[Re-run]

Does your bike still have an oil tank on it? Those almost look like oil lines connected together?

[shinyribs]

I had the same thought Re-Run,but I'm not really that up on DOHC's.

Great looking bike.Good luck with it and Welcome!

[alehman]

yup, the oil tank is still there. I'm starting to worry about that little tube now, because I really have no idea where it belongs...

[alehman]

Also, thanks for the quick replies and welcome. I'm really excited to get this thing running.

[shinyribs]

I know on my bike('76 SOHC) there is a breather can with a filter element under the battery box. The tube you are holding in your hand looks like the tube that-on my bike-runs from the valve cover to he breather can. I also have hoses the run out of the engine/transmission that connect to that can also. Maybe that is where they belong??? I'm really not up on DOHC's at all.Do you have a breather can on the bike? Maybe it has been bypassed or taken off?

[cdaiscool]

Yes, DOHC does have a breather. It's around the battery. Can't remember if just under, or right behind.

But we do have one.

[alehman]

The breather can is definitely there- it's right under the battery. It has two ports on it to which tubes can connect. Mine already has one tube connected that looks like a plugged-up drain line because it's purposefully routed down to the bottom of the bike by the brake pedal. the other port looks like it fits the tube that I'm holding in my hand in that photo, but I have no idea then where the other end of that tube would go.

Does it relate to those two connected tubes? cdaiscool, how are the tubes on your bike routed?

[shinyribs]

This might help some... http://deuceswild.yuku.com/topic/1040/Jeremy-s-1981-CB-750-K?page=3

[alehman]

also, shinyribs, I haven't noticed any tubes coming from the valve cover on my bike (there might be-- I could be mistaken) and that tube doesn't look long enough to span that gap. I found it at the way back of the transmission, wedged near the rear brake light switch/drain tubes, and I'm not sure if I pulled it out from somewhere it was connected or if it was just put there by the previous owner to hang out. I'm inclined to think it was wedged there, just by how it felt as I pulled it from out of there and the fact that I really can't see anywhere there where it could go.

[alehman]

as mstspot says in that link, (referring to the little filter setup he put between those two connecting hoses)

"That breather has a story all it's own. It was one of those gimmick devices that you installed on one of your car vacuum lines so that you could get like 200 miles to the gallon. :) Someone gave it to me back in the late 70's and I've been waggin this damn thing around all these years just knowing that some day I was gonna use it for something (yes I'm a pack rat!) that cap pops off and it has a little piece of foam that can be taken out and cleaned. To answer your other question, I don't know why you would need two filters since on a stock set up they both tie into one location to the air box. All those vents are for are to let out ring blow by so that pressure doesn't build up in your crank case. The EPA had Honda route it back into the airbox where a separator let the condensate go to a catch tank and the dirty air was used again. In reality those hoses could just be routed back and down so that they keep the rain out, when the motor is running you will have a slight positive pressure so they blow and not suck so the chance of dirt getting in there is slim, but that just doesn't look as cool as a little filter now does it. Ha It will need washed at each oil change"


That answers the question about those tubes. I don't have the stock airbox so I don't really have a choice of routing them into the airbox. As he says, there is a separator in the airbox that separates gunk and air and allows the air to recycle into the engine while the gunk goes to the catch tank under the battery. I'm starting to think that the smaller tube I found was the line for the gunk that came out of the separator, and which led it to the catch tank. If I'm correct about this assumption and not going to used the airbox, then I won't need either the little tube or the tank. Thoughts?

[shinyribs]

That sounds about right. Hard to say for sure. All of the hoses on my bike that are just hanging down for vents wedge right in front of the swing arm. Seems like a bad spot since if it pukes something it is right in front of your rear tire.Maybe that is why the PO had it over to the side?

[cdaiscool]

Next time I take my seat off, I can look closer at the hose routes. But it's a SERIOUS PITA for me, as I have bags that hold the seat up.

Per my dad, that crankcase box needs to be emptied from time to time. It's a maintenance item.

[alehman]

Thanks cdaiscool. One more mystery tube I actually wanted ask about. Besides the fuel line, my carb has another thinner tube leading from it (I can get pictures tomorrow when I'm able to photograph it) which doesn't seem to go anywhere. Again, I don't have the airbox for the bike so it might be intended to lead there. any ideas?

[cdaiscool]

Nope. Download the FSM off the home page. Otherwise, Hellboy or whatever the other guy's name is also has an '81 K. So he can possibly answer things I don't. He's got a build thread, check it over and check it out!

[tangylee]

alehman, those tubes that connect into each other are the two crank case breather hoses. you are correct that they connect to a T piece that then goes into the airbox with positive pressure, bringing oil with it. this then goes through the airbox (the air gets burned in the engine) and the oil goes down a port and into tubes directed under the battery, then through a box which separates the thicker material?,then it drips below your bike. at least i think this is how it works. i just got an airbox and think i figured this out. the previous owner of my bike put air pods on and did the exact same thing with my crank case breather yours are now. someone else on this forum mentioned when these breathers are clogged, your engine will find its weakest point and make that its new crankcase breathers

mine rained oil until i found this out and replaced some gaskets.

good luck