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I recently acquired a 1981 CB750K with 1900 miles on it and had very little trouble getting her up and running. Now I'm having an issue getting it to idle happily so I've begun the task of tracking down vacuum leaks. I have found a tee between carbs 2 and 3 and can't seem to find an answer about what it's supposed to connect to...the Haynes manual I've got calls it simply "t-piece" with no diagram or description of what/where it connects to. The diagram I have also describes the hose connected to this "t-piece" as "fuel hose"... again with no description or diagram to explain where it's supposed to end up.
I've read quite a few threads on a few sites and have found several answers:
1) It connects to nothing and is simply a vent...I find this unlikely as it has a slight vacuum to it and drawing unfiltered air into a carburetor can't be the right answer.
2) It connects to the fuel diaphragm unit...this option also seems unlikely as the only hose off of the diaphragm unit not connected to anything is labeled "air purge" or something like that and this doesn't match the "fuel hose" designation of the other diagram.
3) It connects to the air box...this seems the likeliest solution but I can't find any ports or hoses on the air box that it might connect to.
Any help would be greatly appreciated, I've been scratching my head over this for quite a while now. Also, here's two photos of the tee I'm talking about.
 
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Check out these two diagrams:
http://www.bikebandit.com/houseofmotorcycles/1981-honda-cb750k/o/m2003#sch44059 (Parts #19 and #24)
http://www.bikebandit.com/houseofmotorcycles/1981-honda-cb750k/o/m2003#sch31409 (nipple on top, middle, of the airbox, Part #2)
Are these the points you are referring to? If these are the points in question, it looks like they mate together. Note that if that nipple, top, center of the air box, Part #2, is a tube into the air box, it would supply filtered air to the carbs.
If that is not what you are referring to, sorry. Just an edu-ma-cated guess...  Testing it with a piece of hose should be quite simple, and quick. Best of luck! Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII
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Administrator
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Welcome to the forum. I am not familiar with the DOHC bikes, but there are some on here that are. However I am wondering if it goes to the vacuum operated petcock.
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Life is not about the number of breaths, you take, but the moments that take your breath away.
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I just can't imagine what could go wrong.
No fire? No explosions? So whats the point of your story?
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1976 CB 750-A X 2
1977 CB 750-A X 4
1977 CB 750-K
1976 CB 750 F
1981 CB 750
1966 Kawasaki SG 250
1981 KZ 750 LTD
1973 CB 350
1979 CM 185 Twinstar
1982 Honda XL 80
South of Eden (Kansas City MO)
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I have a 81 CB750C, I went out and looked at mine. It doesn't have a T just a straight line between carbs. You can probably put a nipple on it and forget it. I tossed the auto fuel shutoff when I got the bike.
Ray
VN Vet 66-68
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It should have a tube go to the top tee and go up over back on the carbs to nothing . I think a breather I can't remember but my don't connet to anything and it shouldn't . But mine is a 1980
1980 CB 750f ss .... Elmira ,NY
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My '82 has the same thing and I scratched my head on this as well after I did a carb clean and rebuild. Mine just has a 12" hose on it that goes through that loop on the rear bracket and over the airbox...to nothing.
It also pass through the small loop on the right side of the airbox bracket, just behind the tank bolt.
As this made no sense to me...I gave up trying to find a different connection.
In 1972 I had to make a choice...quit riding...or quit drinking. God says it's safe to get back in the saddle now...at least I'm still alive...and feel more alive with the wind in my face! Now riding "Helen Black"...1982 CB750SC.
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Administrator
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Prepare grain of salt-
They look like the same type of tee's that are on my older model round-top style carbs.IFit is the same setup they are just vents for the bowl to "feel" atmospheric (typical outdoors)pressure. IFthat is what they are then they just are vents and will run to the loop at the rear of the engine case
IFthat's what they are. I could be wrong,but all carbs gotta sense outside pressure to work
-take grain of salt now
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Administrator
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I don't remember seeing this piece on my old CB750L. It did have the fuel manifold valve, but if I recall that had one line hooked to a vacuum source, one fuel line in from the petcock, and one fuel line out to the carburetor fuel input rail.
Interestingly, my VT700C has a similar line between its 2 carbs, which is a pressure balance port. It's connected to a long tube that's open at the other end. If the end of that tube is in the path of moving air, the vacuum balance changes, and the bike gets all weird-running (normally around 40-45mph and faster).
I'm sure some digging around on the microfiches at bikebandit.com or maybe in the FSM will locate it. If it were me, I'd ride a bit with the port open, then cap it off and ride it again. Whichever makes it run better, go with that.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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I believe its used as a part of the auto petcock. I recently removed my autopetcock and my bike runs much better.
If you have removed it you can leave it open or stick a hose on it and run it into the airbox or put a little filter on it if you want. I'm running mine open and haven't had any issues.
I think stock was actually just a hose running to the airbox.
Don't cap it. There is another port that should be capped without the autopetcock on the front of the carb block between carbs 1 and 2, kind of on the opposite side of the fuel port.
I have the same bike as you. Enjoy!
1981 CB750K with 900 cams
90K KM's, rebuilt head, rebuilt carbs, upgraded valve stem seals
My wife's recipe website that I'm trying to help promote: Strawberries for supper. Yes, I am a lucky man.
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Thanks to everyone for the responses, sorry I haven't kept up...the last few days have been hectic. I've finally got the old girl running more or less smoothly, so I'm taking her to work tomorrow to perform an experiment. I have attached a hose to the tee and jammed a bolt in the end of it...I'm going to ride to work with it in and home with it removed. I dismantled the carbs again and realized I never cleaned the fuel supply lines, so there was plenty of sludge inside the bowls and jets again. When I put the rack back on the bike the vacuum was gone...I'm hoping it was a matter of disrupted air flow that has now been fixed.
I'm really starting to think that it is just a dead-end hose, because I've seen several diagrams and all of them show it as just that...a hose that leads to nowhere important. I'll be sure to let everyone know how the trip goes, I'm very excited to get her out and on the road!
Also, I'm sure it's not part of the fuel diaphragm because that hose is still connected and in place. It's part #4 in this diagram and comes from the front of carb #2: http://www.bikebandit.com/houseofmotorcycles/1981-honda-cb750k/o/m2003#sch45654Speaking of the diaphragm unit, does anyone know if leaving it on but re-routing the fuel lines to bypass it will cause a loss of vacuum pressure? It seems to be a dead-end and I don't feel any suction from the barbs for the fuel lines but I'm still having some odd idling behavior that seems like loss of vacuum. I've checked all the boots and haven't found any that seem to be leaking.
Thanks again for the help everyone, I'm not as well-versed in small motors as large so this has been a nice learning experience for me.
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I'm kind of surprised that you decided to block it, given that everyone here told you that keeping it open is the way to go.
but hey it's your bike, you can do what you want to it.
1981 CB750K with 900 cams
90K KM's, rebuilt head, rebuilt carbs, upgraded valve stem seals
My wife's recipe website that I'm trying to help promote: Strawberries for supper. Yes, I am a lucky man.
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Float bowl vent.
Do not plug it up or your engine
will not get any gas. It would be like putting your thumb over a straw.
On a Roadstar Adventure.
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You cannot reach some people.
If it had suction then it should be capped.
If it had no suction it was a float bowl vent.
End of story.
On a Roadstar Adventure.
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The float bowl vent just vents to atmosphere (no where).
On a Roadstar Adventure.
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BTW if you run it to the airbox the engine can suck fuel right out of the float bowl!
It was supposed to have a little metal clip or device over it to prevent that.
I would just hang it overboard out of the path of the wind. Like the old days.
On a Roadstar Adventure.
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Administrator
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It looks like a float bowl vent to me,too.But he did mention that it had suction on it which sounds wrong.BUT,there is a possibility of vacuum being present on a float vent while the engine is being revved.It would be slight,but you could detect it. I'm going with vents on this one.I know it is a completely different carburetor than my round tops,but the basic layout it the same and those tee fittings are bowl vents on mine.
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All of the float bowls must have a vent or no fuel will can get sucked into the engine.
It is like putting your finger over a straw filled with fluid. As soon as you take your
finger off of the end of the straw the fluid falls out. Same thing.
On a Roadstar Adventure.
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It is connected alright . Connected to the atmosphere to vent.
On a Roadstar Adventure.
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Well running it plugged/unplugged made absolutely no difference whatsoever, and the vacuum seems to be gone now, so I'm taking everyone's vent idea as the right answer. I also ordered sync gauges a few days ago and I'm going to wait to do any more puttering or tinkering until I know the carbs are back in sync the way they're supposed to be. Will keep everyone posted and have pictures in the next week or two.
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Administrator
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Pictures! We always want pictures!
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