Topic: What is this tube? 1978 CB750K

[dthohler]

Hey guys,

I have been over the parts fiche for a while now, and cannot figure out what this tube goes to. On the number 2 carb, there is the inlet for the main gas line. But what is the smaller line (see picture) that connects a bit above it? It connects between all four carbs, where does it go?

Another question- the only wire not plugged in anywhere is the green one also in the picture. It comes out of the wiring harness next to the horn wires, but I cannot seem to find a home for it.

Anyone know? Any help is appreciated.

[ZanVooden]

Atmospheric vent.

[the technological J]

Green is ground so it may be an extra

[axehole54]

I also had the extra green on my K8...

[bollingball]

Looks like everyone is correct. Don't plug up that hose it need to vent for gas to flow properly and it needs to be up above the carbs I just tywrap mine to the frame.I also put a weedeater fuel filter in it to keep any crud or bugs out.

                                      Ken 1978 750K

[BobbyR]

The hose is a vent. The green wire has always been a mystery. It is not being used. It may be for another bike they used the same harness on, or the next bike.

[TwoTired]

The hose supplies the pressure to push the fuel though the carb jets and into the carb throat.

The pressure results from the column of air above the tube reaching up toward space.

[TwoTired]

The hose supplies the pressure to push the fuel though the carb jets and into the carb throat.

The pressure results from the column of air above the tube reaching up toward space.

Wait, wait, wait, I thought space was a vacuum??

Pretty close.  But, you aren't breathing in a vacuum.  And the earths gravity to hold air against it's surface has a diminishing effect as you get closer to space.

Want to lose weight?  Launch yourself into space. BYOBA   

[dthohler]

Thanks for the replies everyone, I guess my answer is to not do anything to either loose end. I suppose my initial concern was based on the fact that it looks like it was attached to something before the carbs were pulled, guess not...?

[cookindaddy]

The "extra" smaller hose on the 78K carb is a breather as was mentioned. What you don't have is a small screened canister that plugs into the end and (I guess) prevented larger particles in the air from entering the carbs. It is not on the fiche. I puzzled over this vent also when rebuilding mine.

+1 on the "extra" green wire. It seems to be a spare. not connected to anything on mine either.

It is a great bike. I hope you will enjoy it.

[MHansen105]

So now that I know that this tube is a vent for the carbs, what happens if it is clogged up as mine was?

I ask because I've had a problem where after the bike is well warmed up, the power will start to die like the bike is getting no gas.  After a minute or so, it will pick up again and work well for a short while, then the power fades again.  This cycle repeats until I shut the bicke down and let it cool off.

I've just cleaned out the gunk and I'll see what happens.

Monte

[TwoTired]

So now that I know that this tube is a vent for the carbs, what happens if it is clogged up as mine was?

The "Vent" port is in truth an pressure port to let in atmospheric pressure.
The PD carbs have multiple sources for this pressure, as well as balance tubes between carbs.  Another pressure source is the overflow stand pipes, whenever they are not blocked by serving actual overflow duty.

I ask because I've had a problem where after the bike is well warmed up, the power will start to die like the bike is getting no gas.  After a minute or so, it will pick up again and work well for a short while, then the power fades again.  This cycle repeats until I shut the bicke down and let it cool off.

I'd look toward carb sync and/or Idle mixture screw settings to abate this issue.  (Assuming the carbs are, in fact, clean inside).

Cheers,

[MHansen105]

Thanks TwoTired.

I was hoping it would be something simple.

I've had th bike about two years, but have not had time (or a need, as far I knew) to do the carb sync yet.  I'll let y'all know how it goes.

[bollingball]

MH  Also chhech your vent in the gas cap.

Ken

[MHansen105]

bollingball,

Good advice.

I did check that vent, as well as the vent tube that runs under the tank, all clear.

My vent hose is really in two pieces, and has deteriorated at the connection, so I'll replace that.

You can see the join in one of the pictures I've attached.

Also does anyone know what, if anything, is supposed to be connected to the fitting on the air box?  I seem to recall earlier car engines haveing a line running from the crankcase vent to the air cleaner, would this be the same?

The carb overflow tubes are pretty well shot, so I'm planning on replacing them.

The carbs themselves look O.K. but I don't know when, if ever, they were cleaned last, so a carb rebuild is going on my to do lis.

Thanks for all the advise!

Monte

[MHansen105]

Guys,

O.K.  I've answered the question about the fitting on the air cleaner.

The Service Manual shows a hose from this fitting to the Breather Separator.

I attached a photo of the separator.

You can see the connection point.

Obviously, my hose is missing.

Thanks again.

Monte

[Drewster]

Gees, I was just going to post about that carb vent.  Been trying to get my '77 750K to run right, and have had the carbs off four times to clean and try differnt jets.  Overdrilled the slow jets to .058mm. (have 42's on order)   Problem may have been this vent plugged with a screw at the top of the hose by po.  Similar problems running and badly fouled plugs.

[bollingball]

I did check that vent, as well as the vent tube that runs under the tank, all clear.

If you are talking about that hose that connects to the fuel tank that is a over flow hose in case you spill fuel or rain or wash water gets under the big door.
it just goes to the swing arm area.

Ken

[mono]

The hose supplies the pressure to push the fuel though the carb jets and into the carb throat.

The pressure results from the column of air above the tube reaching up toward space.

Wait, wait, wait, I thought space was a vacuum??

hahahahaha

[vistirus]

I thought what everyone is referring to as the "carb breather" (small tube behind the fuel line connection) plugged in to the top of the air box.  There is a small male connector on top and the tube has a hose clamp on it. If it is supposed to just stick straight up, why the connector?

Can anyone advise on this?  I am not sure.

[vistirus]

Sorry, I meant to say why the clamp on the small breather tube?

[TwoTired]

I thought what everyone is referring to as the "carb breather" (small tube behind the fuel line connection) plugged in to the top of the air box. 
No. The air box does not supply the necessary atmospheric pressure to the fuel in the carb bowls.  The filter's top connector supplies negative pressure and a destination to operate the the engine breather/separator.

There is a small male connector on top and the tube has a hose clamp on it. If it is supposed to just stick straight up, why the connector?

The connector simply added and extension of that tube to another that routed behind engine and in front of swing arm.
I expect you'll find the routing a "challenge".  A service connector likely allowed it easier to provide some other maintenance.  Or, perhaps it was part of a production "expedience" mechanism.  Are you certain the observed tube short segment was originally supplied by the factory?

[vistirus]

I am attaching a picture of  the tube in question on my bike. A tube comes up from the "carb breather" . There is another tube that comes off the top of the air box - it is connected to a right angle male plastic fitting on the  top of the box and points  toward the front of  the bike.  This tube has another male connector in its' end allowing the joining of another tube to it. It leads right to the "breather tube" coming out of the carbs and I have connected the tubes together so that the breather tube actually leads back to the breather box. Is this incorrect?  If so, what is the tube coming out of the top of the box for?  Am I missing something?
Should these tubes be left open?
 

[vistirus]

Sorry, I forgot to mention it is a '78 cb750k and I am the original owner.  There have been no modifications and the all the tubes and connectors you see in the picture came on the bike from the factory.

Thanks!

[vistirus]

Well, one more post about this carb breather tube. I was wrong and this thing was driving me crazy, so I pulled the air box off.  The tube doesn't run to a right angle connector.  It is just held in place on TOP of the air box then continues a short way to the rear and ends. It is in effect a long tube from the carb breather that just remains open.