Leaky Gas Cap

[DB Cooper]

I have an 88 FJ, and I was wondering if anyone else has a problem with a leaking gas cap. It only happens when I fill it more than ¾  full, and when the engine is warm or the bike is in the sun.  It will leak while the bike is up on the center stand, not just on the side stand.  It almost seems that the vent isn't working, but yet it does have the infamous FJ whistle. Could the vent only be partially working? The gas cap gaskets are in good shape, and does seem to come up through the key hole, which also indicates a build up of pressure.  Any suggestions?  
Kevin

[RichBaker]

3/4 full by the guage or actually looking inside?  I don't see how it would get high enough at 3/4 actual level inside the tank to be able to get out the key hole.... the only time I have had a similar occurance was when I filled the tank, and then parked in the sun outside the gas station for a few minutes. The sun heated the fuel and it expanded.....

[DB Cooper]

3/4 full by looking inside (about 1 1/2 " below the neck) the gauge actually shows a little over 3/4. 
Does anyone know what would happen if the 2 flapper valves in the gas cap were removed? Would it let more air out of the tank  and reduce pressure a little. Or it be too much air flow and just let fuel out through the vent anyways. Just wondering to try this, fill it up and see if the leak persists?
Kevin

[Firehawk068]

Kevin,
I have this same issue with my '90
If I fill it up close to full, and then run it(engine gets warm) it will wistle as built-up vapor pressure escapes from the vent, but will also push fuel out through the key hole.
Seems to happen worse if i have my tank bag on, so that it gets fuel all over the top of the fuel tank, and all over the underside of my tank bag.......Lately I have been short-filling it if i use my tank bag.

[DB Cooper]

I've noticed that with my tank bag as well, but it will still seep out a little when the tank is full and in the sun without the tank bag. I'm going to experiment with removing the 2 rubber vents in the gas cap and try a fill up and see what happens. I'm just wondering if I'm leaning way over in the corners when the tank is full, if fuel will leak through the vent or not. Will let you know if I make any headway.
Kevin

[andyb]

No, fuel will not leak out from leaning the bike while cornering.  It'll leak if you lean the bike really hard while in the garage with a full tank, but you'll need back surgery after doing that so you won't be able to ride for a few weeks anyhow.

Pulling the vents won't make a difference to what you're experiencing, but I've been wrong before.  Certainly won't hurt to remove the vents.

[Pat Conlon]

Hey Kevin,
1) Be sure to fill your tank with the bike on the side stand.
2) Never fill your bike on the center stand.

If you fill your bike on the center stand then lean the bike over on the side stand, you will leak gas, guaranteed.
Especially if you have your rear end raised via shorter dog bones or adjustable shock.
Empty the tank and fill it and check the volume (in gallons) it takes to fill the tank. Fill the tank to the level specified in your owner's manual. It is very common to have a gas gauge mis-calibrated. Adjustment by bending the float rod will correct your gas gauge. The gas cap flapper-ectomy is recommended to relieve pressure buildup, but will not solve your leaking issue.

HTH

[DB Cooper]

So, gas cap flapper – ectomy successful - very easy to do BTW.  I have yet to fill it, it's raining outside right now. I am going to try and fill it reasonably full, take it for a run and warm up the engine. Then I'll leave it in the sun to see if the gas still leaks. Already I'm thinking this will significantly reduce the pressure, and hopefully keep the fuel from leaking through the key hole.
Will  post the results as soon as I know something.
Kevin     

[DB Cooper]

The latest.....
The flapper-ectomy worked. I filled the tank last night, to within a 1/2' below the filler neck.  I locked the front brake and rocked the the bike back and forth to near the limit of the fork travel trying to splash fuel up through the key hole.Then I got a buddy to help me lean the bike over as far as we could, and no leak. Then I went for a 125km ride with the boys through the Gatinea Park twisties, and a couple laps around the local highway ramps, and not even a drop of fuel, or even the smell of gas. The air temps were not as warm as usual, but the engine was producing enough heat that would normaly cause presure to build up in the tank, and force fuel though the key hole. And this is for Alan (Firehawk 068), this is all with the tankbag on!
Kevin

[Firehawk068]

Thanks Kevin, Great job
Glad this worked for you.......I was suspecting the flapper vent issue on mine as the reason it pushes fuel out through the key hole......I will go ahead and remove mine when I get some spare time to work on her...