Z1OwnersClub GB

New cap, repro colour tank all together and with fuel. Run the bike for a while when shut off their is hissing via the cap. Obviously under vacuum, cap is exactly same as OEM so to the tank.
I can see where there is a vent just under the cap and there is the o-ring around the fuel inlet. So how does the tank vent if it's completely sealed with the wide O ring?

There is an offset hole arrangement in the fuel cap as far as I have seen. But you would need to take the cap internals apart to check that.
Hissing could be air going IN or OUT if there is any expansion going on.

The test I was advised on is to fill tank, close cap and then run the fuel tap into a bucket or similar via a long tube.
Initial flow rate should be pretty well maintained until you get near reserve.
If it does this it is breathing ok in the cap - if not lift the cap and you should see flow increase significantly. That will prove the cap is at fault and not a blocked filter in the tap etc.

Good luck with it.
From Mark