1981 CB750K Fuel System

[tektrix]

I'm fixing up an '81 CB750k, and the overflow tubes that are supposed to run from the bottom nipples on each of the four carbs are missing.

Two questions:

1. Are there any specific tubes I need to replace these, or will any standard line work? If I do need a certain part, I have no idea where to find these.

2. Where do these tubes run to from the carburetors? I've seen schematics of the tubes connecting to the bottom nipples, and then going toward the back of the bike, but I have yet to see a chart or any information as to where they go after that.


Also, if anyone is familiar with the automatic fuel flow system that was implemented with the 1980's 750Ks, any information they could provide would be great. What I'm looking for is a chart or picture showing exactly how it sits above the carbs on the right side, all of the tubes and fittings, and where those tubes go to on both ends.


Thank you all for your time.

-Tek

[TOOLS1]

Welcome to the forum. I do not know about the automatic fuel flow system, but the overflow tubes are just 3/16 vacuum/gas line. You can get it at any auto parts store. They just route down behind the rear of the engine crankcase.
TOOLS

[LukeM]

Tektrix-
Are you referring to the vacuum operated fuel valve for the carbs?

If so, there is a vacuum line that runs from the #2 carb to the valve, which supplies vacuum while the engine is running.  As long as there's vacuum, the valve is open and fuel flows from the petcock through the valve and off to the carbs.  Once the engine stops, the valve closes and fuel flow stops. It's a nice safety feature should an accident happen: the engine stops, and so does the gas.  It's a bit of a pain if you haven't ridden the bike in a while and the carb bowls go dry.  Then you have to crank and crank just to get gas to the carbs.

It's a fairly easy install: There's a bracket that holds the valve just behind carbs 3 and 4 on the right side of the engine. There's a vacuum line that goes from the valve to (I think) the #2 carb, the inlet of the valve goes to the petcock, and the outlet goes to the fuel rail for the carbs.

You can bypass this valve, and run the petcock line straight to the carbs, but you need to get in the habit of turning off the petcock when you shut down the bike.  If you don't, it's overflow city.

Luke M

[sgtslag]

If your bike has a petcock, in addition to the vacuum valve, check to see if it has a "Prime" position.  This is used to let fuel flow freely, to fill the floats, as mentioned, when the bike has been sitting a long time.  Be sure to move the petcock back to normal, after starting, though, or you will experience overflow of fuel, as previously mentioned.  Cheers!

[Tektrix]

Thank you all for the replies so far. I have Keihin carbs by the way (not sure if that makes a huge difference).

@TOOLS1,

When they go behind the engine crankcase, what do they connect to? The diagram I've seen for the model bike I have ('81 CB750K) shows them running toward the back of bike. I've also checked out some photos online taken from the side, and they seem to run under the air cleaner to somewhere. It seems they would have to connect to something.


@Luke

Your info on the fuel system is quite helpful. I think it may be the #3 carb, as it has something extra on it that the other three don't have.

[LukeM]

RE: tube routing.  They don't connect to anything. They are supposed to take the overflowed fuel and drop it on the ground. Route the open end of the lines between the engine/trans and the airbox, but don't let the ends drip fuel in front of the tire.  That could be dangerous. I think there is a hose routing diagram in the FSM.

RE: vacuum line connection.  I knew it was one of the carbs. Not having that hose connected would be equivalent to a vacuum leak, which would make it run bad.  You'd hear it hiss, and if you put your finger over the end, the idle speed would increase a bit as well.  Glad I could help.

Luke M