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Is the fuel tank supposed to be airtight? Because when I've been riding for awhile and the bike is hot, I can hear gas vapor seaping out of the tank after I stop.

guitar guru wrote:Is the fuel tank supposed to be airtight? Because when I've been riding for awhile and the bike is hot, I can hear gas vapor seaping out of the tank after I stop.

no, gas tanks are not airtight. In fact, the older ones wouldnt work if they were as the gas comes out of the tank on gravity alone. It would have the same effect as filling a straw with water and then putting your finger over the top.

usually the gas caps are vented to let air in and out. What you are hearing is probably normal.

No. ... and chances are no you don't.

Think about it this way... you start with a full tank... you burn off that tank... if the tank was airtight you'd have a tank full of nothing, right? Which means you'd have roughly 14PSI of pressure (the normal pressure of air all around you) trying to squeeze the cap closed, the tank sides in, and so on. Not only would it be hard to get fuel out of, it would be hard to open to get fuel into and it would need to be really strong to prevent crushing.

So they have vents to allow air in to replace the gas that went out. It also allows air out if the tank heats up... but mostly it lets air in so fuel flows (think water cooler... water stays up... drill a hole in the bottle and where does the water go?) and the tank doesn't crumple like a beer can.

If you smell fumes try not to smoke around them. KABLOOEY!

An R6 just came into the shop this week with the fuel tank sucked in around all four corners. Found the vent line pinched. Luckily we talked Yamaha into getting him a new fuel tank eventhough it was out of warranty. Yea, us.

The R6 has a fuel pump, eh? With gravity feed the worse that can happen with a plugged vent is that the engine quits. If you've got a fuel pump sucking away, your tank starts imploding.

To the original poster : I'd check out the tank vent if I were you. If you can hear air being sucked in after you stopped the engine, then you could very well have a partially plugged vent. If it gets completely plugged, things are going to get quiet.

I can't believe how dumb some of you guys are! Why is any gas tank vented? When you open a can of canned milk and you punch one hole in the can the milk won't come out, do you know why? Air pressure must enter the can to push the milk out. A fuel pump doesn't pump fuel, a fuel pump produces a lower air pressure in the outlet side and the high pressure air on top of the fuel tank rushes in or pushes the gas to equalize the pressure. Air has weight, the gas in ur gas tank flows becuse air has weight. I wish there were some trainned motorcycle mechanics that understood that.

Air doesn't need to enter the tank to equalize the pressure. The tank can also crush. Works fine on bladder-lined tanks... and car fuel tanks if you don't mind destroying the tank (I've had it happen...not a good thing). Should work on a cycle as well and give you that custom streetfighter look in the process.