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Hi,
Im relative new to multi-cylinder four strokes. I bought a '78 yamaha xs400, according to haynes manual and a complete comparision of the wiring diagram i am led to believe its a xs400F / xs4002F.

Now, WHERE ON EARTH do I plug in a carb syncing device?? Also one of the carbs (left-side) has a black 'condom' on the vacuum intake nipple, there was a lesser-gauge black rubber line just hanging from the gas tank petcock, the manual says this is the vacuum (something, i forget exactly what) where does the other end of this pipe go? the other carb nipple???

Also, there is a connection between the two carb air-intake hoses. it has another small rubber tube coming out of it.. but just left hanging. what is this??? Check out the top view i've drawn below

_______
L.Carb -> (_)--------|______/ <-L. Air Filter
|
RUBBER PIPE LEADS NOWHERE! -> ---- | <- Balancing Hose??
| ______
R.Carb -> (_)--------|______\ <-R. Air Filter


There seems to be an inlet/outlet on the engine that this rubber pipe seems to lead into, but im not sure.

Any help is greatly appreciated.. thanks

All my tab spacing for the nice diagram have been screwed up!!! somehow leading whitespaces are omitted.

First off, to sync the carbs you remove the "Condom" from the nipple and plug it in at the two nipples.... Be sure to replace the "Condom" when you are finished. The other hose from the Petcock leads to the other nipple. (Ptecock is vacuum operated)

The hose in-between your carbs is most likely an atmospheric pressure vent for the float bowls ... just make sure it's not kinked. It just needs to hang out of the way somewhere.

there is a possibility that a fuel or vacuum line is leaking or has become disconnected. See, the fuel tap, also called the petcock, when it's set on "RUN" or "ON" only flows fuel when the motor is supplying vacuum to it. You may be able to switch it to "PRI" (Prime) and then find where it's leaking from. This way you don't even have to run the motor to get the fuel to flow.
Good luck, keep us posted.

The two nipples that are in front of the carbs (one of them is capped) are the carb syncing ones (the uncapped one goes to the fuel petcock). The ones on the bottom of the runners to the air-boxes are for the crankcase breather (on top of the transmission).

BTW Hello to all and welcome to the forum. We want to see pictures of your bikes. I have a XS360 in my collection.

I can send you a complete Yamaha Parts Manual for the 78 XS400.
I know the vacuum lines on Yamaha's. I ride a 1983 XV750MK Midnight Virago.
I am a Motorcycle Carburetor Specialist and some of the petcock do work off of vacuum.
Some petcocks have diaphragms in them.
The gas only flows when a vacuum is applied that opens the diaphragm.
The small line attaches to the petcock and the other end to the nipple on the
carb manifold.
These nipples on each manifold is where you hook your vacuum gauges or monometer to
Synchronize the set of carburetors.
I have a couple really nice sets of XS400 carbs in my Shop.
If your Interested. I can send Pics of the Sync screw from a set and send you a Parts Manual.