The tube maybe just a float chamber drain off point. In which case there will be a screw next to it which is undone to release fuel from the chamber. It could be loose or faulty. Be careful because there will be other screws in the vicinity which are adjustments, so don't turn anything unless you're sure. Or if you do turn anything, take care to note how many turns you make so you can retrace your steps.
It is possible the tube you are referring to could be the float chamber vent. Usually there would be a rubber hose leading off under the swingarm pivot area. This is to lead any overflow of petrol a bit more safely and cleanly away. The tubes often get pulled off and never get put back. Or they deteriorate/harden and fall off.
If this is the vent we are talking about then having petrol coming out means the level in the float chamber is too high. Most likely cause would be the little valve pivoted off the floats is not seating correctly, either by wrong assembly or the little rubber seat is past it's best. Another possibility is that the floats have been set too high.
The result will be rich running, which will lessen as you approach full throttle because you are basically consuming fuel as fast as it can get in. The purpose of the vent is to let atmospheric pressure act on the petrol in the float chamber. On the opposite side of the coin, a block in the vent would also cause trouble.
I don't know the bike you are talking about but i'm going from memory of how I seem to recall how jap bikes used to be.
As a clue as to whether the tube we are talking about is the vent, inside the bottom of the float chamber bowl there will be a standpipe (long thin tube), which should project above the level of the fuel in the chamber.
