KZ1000-P7 (1988) fuel cap leak.

There have been several threads on leaking fuel caps.

www.kzrider.com/forum/5-chassis/360869-gas-cap-leaking-gas
69.46.28.106/forum/3-carburetor/587844-t...enting-kz1000-ltd-82

I think I sorted out this issue with my cap and thought I would share. I hope this helps others.

The gas cap is hinged on the top of the tank. The general advice if the cap leaks is to replace the cap rubber tank seal and cap hinge screw o-rings that hold the cap on the tank. I did both of these things and the cap still leaked. Next, I used steel wool polish up the tank opening to make sure the cap rubber seal was making good contact with the tank. Still leaked.

(I wish I was in the habit of taking pictures for sharing. I’m not so sorry, you will just have to put up with my descriptions.)

So I went to the service manual where they said to take the cap apart and make sure the cap breather was clear. To take the cap apart, first remove the two hinge screws. Set aside the screws and the hinge screw o-rings. Replace the o-rings as needed. Then the cap has two screws holding down the cap seal keeper which holds the tank seal on the cap. Before removing those screws, press down on the cap seal. You will notice that there is an internal cap spring that presses down on that seal to insure the seal is held tight against the tank. So when you take those two screws out the cap could come flying apart. So hold the cap seal down, remove the two screws from the cap seal keeper, and then let the spring push the cap seal and the cap seal keeper off. Now be careful, the cap seal keeper includes the latch that holds the cap closed to the tank and a spring that pushes that latch out. If you are not careful you could easily drop these parts on the floor. Set latch and spring aside. Then separate the cap seal and the cap seal keeper. Now look at what is left of the cap. Remove the big spring that pushes on the cap seal. Now again be careful. Look at what is left of the cap. Part of what is left is the cap key lock mechanism. That mechanism includes a little spring or springs, that return the key to the center position when opening the cap. Those springs can come flying out if you are not careful. So remove that spring or springs now. Now at the base of the cap is the cap breather. It is gray metal, with a thin o-ring at the top. Remove the o-ring and set it aside. The gray metal breather is two parts with a gasket at the bottom and in the middle. It is likely on rather snugly, but if you gently work at it, the breather will come off in one piece. I would not disassemble this further. This breather is a bit of a rat race. Clean all the parts on the cap and blow out air path internal to the gray metal part. I sprayed mine out with silicone spray. It needed it because it was rather restricted.

At this point you should be able to reassemble and the cap should work fine. I did that with mine and it still leaked. WTF! So I took it apart again and inspected the whole lot. First, there is an rubber seal between the cap seal keeper at the cap seal. It’s round, but it is not an o-ring. It has a v shape in the cross section. This seal prevents fuel from getting past the cap seal and the cap seal keeper. This seal is held down with a plastic washer that the big seal spring presses down on. That plastic washer will come out. I couldn’t find a part number for that seal so I replaced it with a large o-ring from a kit I bought at harbor freight. Ultimately this mostly did the trick. Why do I say mostly?

Well first, my cap would leak anytime I filled the tank really full. I always fill the tank really full. So when I close the cap the cap seal keeper and the cap latch are both in the gas. Any movement of the bike would have spilled gas prior to closing the cap. After closing the cap, prior to using the o-ring between the cap seal and the cap seal keeper, gas will spill past the cap. Replacing the seal between the keeper and the seal fixed that problem.

The part it didn't fix was that the cap has a tank breather in it. That gray metallic rat race thing I just cleared out. When you close the cap the tank is sealed to the outside world with the cap seal and the cap seal keeper seal. Gas can however get past the cap latch and into the space which includes the lock mechanism and the tank breather. The breather prevents the tank from getting a vacuum lock. Air has to get into the tank somehow or fuel would be restricted to the carbs. So if you fill the tank really full, and park it with a hot engine, the cold gas in the tank which came from station buried tank is now warmed up in your tank by the engine heat and expands. Ultimately it will push out of your breather. This leak will however be much smaller than if you have a bad seal between your tank seal keeper and the tank seal.

So what do I recommend? If you are going to ride right after a fill up, go ahead and fill it very full. The movement of the bike will keep the fuel from heating up too much. If you are going to park just after a fill up, don’t fill it up very full. If you stop for lunch, fill up after lunch.

Two last things. First, there is a seal for the key lock mechanism I didn't mention. If yours is leaking from the key hole you should replace this seal as well. You just need to push the key lock mechanism out with the key as part of the tear-down. I could not find an o-ring in my kit that would replace this part so I left mine in. I wasn't leaking from the key hole. Second, my tank has a rubber tube attached to a nipple on the underside of the tank. I always assumed this was part of the tank breather. Nope. All I can figure is that this tube is for directing fuel that leaks past the fuel gauge sender gasket. That nipple is part of a tray under the gasket and does not in any way go into the tank.

If you are having fuel cap leaking issues, I hope that helps.