XT600 gas cap not venting Tenere tank?
 

I have an XT600 gas cap that I'm using with a Tenere gas tank and have been having problems with venting as the fuel level drops. With a full tank, the weight of the gas keeps the carb supplied with fuel, but as it drops, the vacuum becomes too great. I actually noticed a gurgling sound as I brought the bike out of the garage today; fuel getting sucked out of the carb and back into the tank due to the vacuum! No wonder I was having starting problems.

What can I do? I removed the metal plate around the base and that didn't solve the problem. The previous owner had actually pried it up with washers?! I tried another unmolested cap and had the same problem (I'd open the petcock and fill a jar with fuel, and the flow would slowly diminish, then stop). I could clear out the gasket in the cap as well and have a free-flowing setup, but my concern is that if fuel splashes in there it will block the flow of air.

Are there any available vented gas caps? Or should I tap a hole, put a nipple in there, and run a vent hose?

If the original set up has been bodged and not working (its hard to make out without pics), then id be tempted to replace the cap completly.

Failing that, a hole anywhere with a nipple and a hose will sort your problem out.

I don't quite understand the principles behind original setup and both of my caps didn't work, so I removed the rubber gasket from the inside of the inner cap (not the gasket at the filler neck) and it works now. Gas dribbles out the side of the cap when overturned, but still splash-proof.