78 kz650 hose mystery

Nice petcock! Any details on it?

I believe that hose coupling between the center carbs is your fuel inlet from the tank.

Assuming the tube the arrow points to in the image below is the one in question, you need to keep it on there. It is part of the accelerator pump system. Ed

650ed wrote: Assuming the tube the arrow points to in the image below is the one in question, you need to keep it on there. It is part of the accelerator pump system. Ed

Ed, I agree that this is the tube in question. But that's not part of the accelerator pump. The pump system is all down in the float bowls.

And actually, I don't see any sign of the accelerator pump system on these carbs. While the bike may be a 1979, with the smog ports, those carbs may be older.

But back to this tube. I do not think it's necessary. I think it's part of the float bowl vent system. I believe carbs 1 and 2 are linked, and carbs 3 and 4 are linked, but there is not a vent connection (so no tube) between 2 and 3. This is why there are two vent tubes. When I say vent, I'm speaking of the two tubes that go from the carbs into the airbox, not the vacuum port that used to connect to the petcock.

You can sort of see the vent tubes here:

dohc wrote:

650ed wrote: Assuming the tube the arrow points to in the image below is the one in question, you need to keep it on there. It is part of the accelerator pump system. Ed

Ed, I agree that this is the tube in question. But that's not part of the accelerator pump. The pump system is all down in the float bowls.

And actually, I don't see any sign of the accelerator pump system on these carbs. While the bike may be a 1979, with the smog ports, those carbs may be older.

But back to this tube. I do not think it's necessary. I think it's part of the float bowl vent system. I believe carbs 1 and 2 are linked, and carbs 3 and 4 are linked, but there is not a vent connection (so no tube) between 2 and 3. This is why there are two vent tubes. When I say vent, I'm speaking of the two tubes that go from the carbs into the airbox, not the vacuum port that used to connect to the petcock.

Here's why I thought it may be for the accelerator pump. But this depends on the tube in question being the one shown below with the arrow. Maybe it's a different tube? If so, you are correct that it isn't part of the accelerator pump system. And if the carbs don't have an accelerator pump, clearly it isn't related to it.

In the first diagram below you can see the tube in question with the arrow. Notice there is a line from it to the note "Ref. Accelerator Pump." There are 2 other tubes (between 1&2 and 3&4) that also have the same note. The second diagram below is the accelerator pump diagram that the note references. Notice the tube in question is there between carbs 2&3 and that there is also a tube between 1&2 and one between 3&4. I believe these tubes are used by the accelerator pump to move the shot of fuel from the pump to all 4 carbs. Now notice the 3rd diagram below. It shows the VM24SS cabs without the accelerator pump. You will notice none of those 3 tubes (including the one in question) are present. That diagram is accurate for the earlier non-accelerator pump carbs. In fact, it shows how the carbs on my 1977 KZ650 are configured. Ed

Perhaps #39 in the diagrams that Ed posted.

Good Fortune!

Patton wrote: Perhaps #39 in the diagrams that Ed posted.

Good Fortune!

I think you may be right. There doesn't need to be a #39 tube between carbs 2 & 3, but maybe a previous owner added one. If so, it isn't needed unless the previous owner also opened up those nipples or mixed up carb bodies somehow. Ed

Thx guys. I don't think the bike runs differently one way or another

Sorry for the tardy response I've been trying to deal with rear brake not bleeding. Getting frustrated. Replaced master cylinder but still can't get fluid to flow. One piston on the caliper seemed fine. I don't really want to remove the whole wheel yet to get to the other one.

Those hoses are have to connect to the vacuum ports of carbs 1 and 4, they open the valve and let air flow on the thicker hose into the valve cover.
If the bigger hose is plugged and so the vacuum ports those hoses are useless.