Re: what did you do to your motor bike today?
I've seen a long tube that has some rigidity connected to a Briggs and Stratton old style vacu-jet carb gas tank. It stick up about 5 inches. It goes to the vent in the gas cap. I suppose it is so the vibration and especially with a full tank it does not slosh and spit out the vent cap. It would have to go up the few inches of the hose and so keeps it safer too. The old ones have a carved scalloped out area of the gas tank where the tube muffler is less than an inch above the gas tank.
Heat from the muffler without a fire is still enough to have the gas in the tanks warm up and expand and also more vapors at the top of the column of gas that instead of needing venting inward to the tank it spits/vents gas out the vent right by the muffler. Yikes, but that was then. They're still out there though I got a newer style carb and have it away from the muffler and exhaust manifold.
If for wanting to have a vent shut off when not running the engine, I would see the need for gravity flow bowl type carbs to have a fuel line cut off to be closed at the same time or the vapors creating a pressure above the gas coulmn could push the fuel into the carb and over power the float valve. Then raw gas is flowing out the carb jets and flood the engine and eventually drip out onto the floor. Not good!
One extra thing I have to mention is that traveling over high mountains from sea level even with an extra space above in these spare gas tanks, the road vibration and the effective difference in pressure, it is not safe to do this kind of transport. I can see maybe the steel gas cans mounted outside the vehicle probably have a good amount of strenth and the cap seals, but not these smaller plastic variety.
Last edited by MEASURE TWICE; 05-25-2012 at 02:22 PM.