Hiss From Gas Cap - KZ1000J3

Hi guys,
Been a while since I have been on here.  Just wondering is a hissing gas cap is normal?  Had the bike for 26 years and its earlier days I do not remember this.  This only happens with a full tank after a long ride.

Ride the bike very little - bike runs very well though - and noticed a couple of years ago it hissing for what I believe to be the first time, ignored it and when I opened the cap later I had some spoiled paint on the rear of the fill hole neck.  I then replaced the gasket thinking this would fix it and touched up the paint.  Last weekend when I went out, got back heard the hissing, opened the cap and noticed more vapour or gas migration as the paint was effected again on the neck and a little at the bottom of the neck on the tank. 

Questions:
1. Do you think this is the result of riding with too full of a tank? Gas was filled to bottom of fill hole. First ride after a long storage.
2. Could I have caused this by opening the cap right after the ride (day was hot too) and the pressure forced out vapours and gas (only really visibly btw under the gasket on the shinny silver part of the cap and a little on the lower part of the gasket - perhaps it dripped down?
3. Is the hissing the result of a partially blocked vent?
4. Will pushing compressed air from a can and or WD40 or brake fluid in the key hole help clear any blockages if there are any?
5. How easy is disassembly / reassembly of the mechanism under the cap? I have heard you can run into issues with the caps after this.
6. Best to get a new cap?

BTW, I will check the vent line for a kink although I think it is fine.

Appreciate any thoughts on this. Paint on my tank is nice and want to get this sorted.

Thanks

I would bet on a clogged vent. Could be some small creature has set up housekeeping in the hose. Could also be crud buildup in the cap itself. You should be able to take the cap apart and clean it it out.

Thanks. On the J3, can you tell me what the vent hose connects too?

. 

There is no "Vent hose" on a J3.
I think the hose you are talking about is on the underside of the tank on the rh side? If so that is the drain hose to stop fuel leaking onto the hot engine should the gasket fail on the fuel level sender unit. 
The tank vents though a vent hole in the fuel tank cap. 
It allows air into the tank to stop vacuum fuel starvation as the fuel level drops from the engine running and allows air/fumes to escape caused by heat expansion of the tank contents. 
You'll need to dismantle the cap and ensure all the vent channels/holes are fully clear. Whilst apart you should lubricate all the moving parts. 
Your also over filling the tank by filling to the bottom of the filler sleeve. If the bike were on the sidestand or leaning during cornering then your asking the cap seal to do all the work of stopping leakage. It's not designed to do that it's more of a splash guard when fuel is sloshing around during riding. 
 
 

 Is it reasonable to cover the tank and do this disassembly, clean, assembly on the bike? Screws  - one especially looks really rough, not sure it will come out? What are your thoughts on this? Looks sort of white and corroded. I could also pop the hinge pin out very carefully to remove it. Preference is not to remove entire cap from tank as sealant was used for extra protection against gas leakage.

J1-3 just has 2 screws holding the entire cap on, they are holding the lower hinge plate to the tank. Undo those two and the entire cat and hinge comes off in one piece. Access them when the cap is open. It'll be so much easier with the entire cap off. Those screws are very short so shouldn't be too difficult to undo. 

 

Thanks Warren, I was thinking of a vent hose on a different model.

Haven't seen any diagrams on the cap internals.  Due to the sealant on the two screws fastening it to the tank I am prepared to just protect the tank and take the cap apart and clean it on the tank. Once I take out the two screws what should I expect to see?   I believe the vent is at the bottom of the keyhole in the cap itself which i need to get to. I've attached a couple of pics. Thanks

Not sure if its the same thing but on a normal KZ gas cap you can increase the "vent" hole size to help it vent easier. I use to get a lot of hissing on Summer rides with a hot rod motor until I modified the internals.

The vent hole is small and can sometimes be clogged by tiny spiders! Shown is a KZ650 cap (same as a KZ900 cap)







Its just a cork or paper? gasket inside and its non replaceable so you need to take care you don't mess it up. I guess you could replace it with something else but never tried. Sometimes the gasket is even miss matched over the hole blocking it a little.

One of those parts Kawasaki never intended people to mess with

It helped with the hissing, way less hissing with bigger hole.

I do this to all my gas caps.
 

Thanks, great info. I recognize mine looks different but i suspect the device is fairly simple to work with in that there are not many parts. I think I'll get a spare before I start pulling it apart.

Opened the cap up. And cleaned the vent hole that's integral to the silver cap.  Blew compressed air through it and could feel it coming through the key hole which is good. Question there is a large centre  spring that compresses against a white plastic washer in the metal cap housing that was screwed down  on the silver cap. On the other side of this white washer there is a black rubber gasket that is  groved on one side that fits into the metal housing. I placed the grooved part of the gasket against the white washer as it  seemed to sit better in the housing.  I could not recall how it looked when I opened  it.  Does this sound right or does the flat side of the  black gasket sit against the washer?