Carb Nipple

[Kirk Creswick]

Can anyone tell me what the nipple (circled in red) on this 42B series carb for my 1980 CB750F is for (neither of my shop manuals shows this and I have another 42B series carb that does not have one.

[LukeM]

On my 79 model, there's a vacuum hose that runs from the carb bank to the fuel manifold valve.  As long as there's vacuum, the valve is open and fuel flows to the carbs.  Once the engine stops, the valve closes, and no more fuel.  I think this is what you're looking at.

If you don't have the valve, you can cap off that fitting, and your bike will be fine.  Probably run better too, with the vacuum leak plugged.

Luke M

[Kirk Creswick]

That's it. Models 1981 and later have the fuel line diaphram. Since mine is a 1980 model, it was not shown in the manual. That means the carbs on this bike are not the original carbs.

The carbs I just got from the junkyard don't have the nipple and my tank dosen't have the diaphram, so I'm good!