Topic: I'm so excited!! But what is that tube and where does it go?

[Greg]

Im excited...Thanks All!

So I just finished rebuilding the carbs for the first
time!!!  Bought a 75 750f for a couple hundred bucks,
and am trying to bring it back to its former glory.
And despite all my fears, and a few missteps, the bike
fired right up!  Thanks to everyone that has
contributed their wisdom to this forum, which helped
me do it right!
One question, however.  There is a fat (about 1/2 inch
exterior diameter) tube that feeds into the air box.
I'm not sure where it comes from, and couldn't seem to
find it in my Haynes.  I forgot to connect it, and
after a little while realized that it was emitting a
lot of smoke.  The emissions out the tailpipe were
clean (no smoke). 
What is this tube, where does it come from, and should
it be smoking?
Thanks for all your help, I am really excited to have
done this for the first time and am excited to get
this bike running sweet!!

[Kixx007]

It goes to the cover on top of the motor.  Its the crankcase breather tube.  I asume it connects near the bottom of the airbox, thats how the one on my 550 is.

[Quail "Owner of the comfortable k8"]

On my 78k it goes down to a (Ithink it is a pcv valve) part that is attached to the bottom of my battery box. A hose goes from there to the valve cover.

[Greg]

mine actually goes to the top of the airbox.  I thought it might be the crankcase breather tube, but my haynes says they started putting those in after 77, and this is a 75. 
If it is the crankcase, would it be normal for some smoke to come out?
thanks!

[SteveD CB500F]

No. Smoke from the crankcase breather means you have excess crankcase pressure (ie blow-by past your rings) caused by worn or stuck piston rings. Curb your excitement and keep an eye on it...

[Greg]

uf, that's no good.  I could check the state of the piston rings with a compression tester, right?
thanks....

[SteveD CB500F]

A compression test would be a start.
If your rings are really worn you will be burning oil and notice blueish smoke from the exhausts. This will be more obvious on load, so go out with a mate (following) and whack it open in 3rd or 4th. He should be able to tell you if it's smoking!

PS. It's not terminal. My 500 smokes a bit from #1 cylinder - I'll get around to it one day...

[Greg]

well that's good to hear.  I was revving her pretty hard yesterday and she didn't blow any smoke out the pipe, so if the rings are worn, then they aren't too bad, right??
thanks for all that offer advice!

[Terry in Australia]

The big fat hose connects to the top of your valve cover thingie (tappet cover/cam cover/rocker cover, whatever you call it) and feeds a little oil mist back into your airbox, which is recycled back into your engine via the carbs.

Connect it up and see if your exhaust smokes, if it doesn't, put your happy face back on, and enjoy your ride! There is also a crankcase breather at the rear of your engine that feeds into your oil tank too. Cheers, Terry.

[El Taco]

BE GLAD IT SMOKES.

I bought a locked up but complete CB350 for $5 once. Got the motor unlocked in three days, and took off on it. Never thought to check that PCV tube.

Well, a small insect crawled up in the hose and made his home there in the 7 years this guy had this bike in his back yard. It was plugged solid. I came to a stop and heard a hissing noise. Didn't think much of it. Should have.

Long story short, I put 5 quarts in it in 20 miles. Make sure that hose is clear, smoking or not.

And my CB500 has a little bit of blow-by. Not a big deal.