TOPIC:

There's been a fair bit of chatter about issues with the venting of the fuel tank and I thought maybe it would be worthwhile to go over the gas tank vent system and some of the problems one could find and simple cures.

At the bottom of the tank on the right hand side is a tube and attached to it should be a 7mm hose with a "fuel tank breather" on the end of it and a hose on the end of the fuel tank breather going down and venting below the frame. This breather is a one way check valve which allows atmospheric air to enter the bottom hose and through the breather flowing upwards towards the fuel tank. One problem that could prevent the fuel tank from getting air is a breather stuck closed or installed flowing the wrong way. The arrow on the breather should point towards the tank.


Picture of the tube on the bottom of the tank. This tube runs upwards inside of the tank and exits the fuel tank in between the 2 screws which hold the fuel tank filler cap on. As a test you can remove the hose at the bottom of the tube and blow air into the tube and check to see that the hole at the filler cap mounting bolts vents the air.


Picute of the vent hole which is the other end of the tube at the bottom of the tank.

The Kawasaki filler cap has a vent system cast into the cap with a tube that lines up with the vent tube on the fuel tank and there should be a rubber seal like a rubber flat washer under the bracket which holds the filler cap on.


You can see the vent tube in the casting between the 2 mounting bolt holes. This tube exits the filler cap underneath the spring loaded seal and appears to be about 2mm in diameter. If you apply compressed air to the vent in the casting while holding down the spring loaded seal, you can verify or blow clear the vent in the cap.


A very small hole which could easily get plugged. This hole is where the atmospheric air exits the venting system and enters the fuel tank. No air into the tank, no fuel out.
This is a pic of the aftermarket fuel tank filler caps that are readily available on Fleebay. Note in the picture the lack of the vent casted into the filler cap.




This doesn't necessarily mean that these aftermarket filler caps are not vented. They may be vented through the top of the filler cap, I'm not sure, but they are definately not designed to vent the way that the original ones were.

Looking at the images of the aftermarket and the OEM fuel caps I would think that you'd get fuel splash because there's nothing there to draw the fuel down the overflow tube correct me if I'm mistaken

stocktoy wrote: Looking at the images of the aftermarket and the OEM fuel caps I would think that you'd get fuel splash because there's nothing there to draw the fuel down the overflow tube correct me if I'm mistaken

You're losing me here Stocktoy. That hole in the top of the tank is not part of an overflow system. And when all of thr right parts as designed by kawasaki are in place, fuel nor air can go "down" that hole and out to the ground because the fuel tank breather is a one way valve only allowing air into the tank.

The fuel tank breather is the item below item #2

I also thought that air could move both ways with the venting system to release a build up of gas pressure when the petrol gets sloshed around. Maybe the valve you mention isn't working then, I also noticed that when I replaced my cap similar to one shown I got loads of petrol pushing past the main filler cap seal and on to the top of my tank. This I though must have been due to a build up of pressure ( with a non vented cap), I put the original cap back and made sure air could pass through the tube and all is good again. The main reason for trying the after market cap was the chrome on the original was poor.

I believe the breather could cause all sorts of trouble. I'm sure the fuel in the tank when heated up under the sunlight will vapor off somewhat and if it can't relieve to atmosphere, could build up pressure and possibly overcome the floats in the carbs and push fuel (overflow) the carbs. This is a complete guess at this point. The breather may be an over pressure relief device as well? I find it hard to believe that Kawasaki didn't think of pressure buildup in the tank under sunlight or heat from the engine.

Just looked at the drawing showing the in-line device on the tank vent hose. If my bike ('80) ever had this fuel-filter "look-a-like" device I can't remember it, but have not had anything like this on my vent line for as long as I can remember.
I don't recall where I read this, but I do recall this topic somewhere (could have been another site and another bike) and My recollection is that it was to prevent a "lock" in the hose if road-water got up into the draining end of the hose which was theorized to cause a vacuum in the tank which we know will cause fuel starvation. So I'm suggesting it is not a valve assembly of any type. It simply provides a volumous area for interrupting the "sucking-in" of water. AND Yes I would have to agree - The fuel tank would have to be emptying fuel at a pretty good rate to pull water up into the vent-hose. Think about it: The vent hose must let air in as fuel is depleted and like wise must let air out in the case of expansion.
If there is a check-valve component in the device it would have to have a VERY lightly sprung mechanism to allow for venting either way. And that sounds like a problem area if dirty
Anyone willing to cut one open to see how it does work?