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Hey everyone, I am finishing an overhaul on my '84 ZN1100, and seem to be missing a vent tube.  After rebuilding all 4 carbs, cleaning the inside of the fuel tank and rebuilding the level sensor the bike is running like new again.  I had to redo all the emission lines to the evaporation canister and fuel tank vents, but there does not seem to be a tube connected to the vent cap on the fuel level sensor.  If you have seen one of these, it is a square plastic cap that snaps over the base of the sending unit.  The cap has a vent nipple on it.  Does anyone know if there needs to be a vent hose on this to vent fumes or can it just vent under the tank?

I don't think it's a vent but a drain, don't know re your model but on the level sensor cover for early 80's 1000 j's there is a tube connected that runs back under the tank and down the back of the ehgine by the swingarm pivot.
I assumed its there in case of sensor level gasket leaks to drain fuel away from hot engine components. 
It's shown as 2 pieces on the diagram to denote its longer than shown on the diagram. 
 

I don't know but would agree with Warren. Any venting would need to be done from the top of the tank

Thanks Warren, I agree with you and Wookie58 - It's a drain, not a vent.  There really shouldn't be any fumes emitted from the sensor, but if it ever leaks, a drain tube would be important.  I had already installed a tube exactly as you described, and was wondering if I should leave it there.  In my mind, everything needs a reason to be, and I could not see a need to vent the sensor, but now it makes complete sense.  My fuel tank is only slightly different from yours, but essentially they have the same components.  Thanks for such a detailed reply.  

Make sure to run the drain tube to the SIDE of the tire, not right in front of it, for obvious reasons...

Thanks, I'm pretty sure I have it on the right side, but I'll double-check that it's clear of the tire.