A gas tank needs a vent. Otherwise, as the fuel leaves the tank and is blown up in the engine, a vacuum is created and eventually the fuel pump won’t be able to suck hard enough. Try this: plug your nose. Say “wukka wukka wukka!” No seriously, plug your nose and breathe through your mouth. Then cover your mouth with your hand. Not for very long though. Now release your nose. Your nose is like the vent valve, your lungs are the engine. Oxygen obviously represents gas. As your lungs try to get more air, pretty soon the reserve in your mouth runs out, but as soon as you open the vent (your nose) the engine runs smoothly. If this experiment didn’t work, see a doctor – you’re not plumbed right.
Moving right along, here’s how I vented the tank.
Revision 1 was slipping a rubber hose over the barbed end of the gas tank vent. Revision 2 was copying Cam, who slipped a short section of rubber hose over the barbed fitting then transitioned to stainless hard line. Here’s revision 3:
The brass piece on the right is the rollover valve/ vent. You can see the barbs on the top of the fitting where hose normally slides over. I’ve ‘toned down’ the barbs. The “rollover” portion is the little ball inside the lower portion of the brass piece, which in the case of being greasy side up, will prevent fuel from leaking out the vent valve.
The 3/16″ stainless line *almost* fits inside the barbed part of the brass valve, so I machined it down just a bit so it slips tightly in. Remember, this line is mostly for vapors, with the occasional fuel sloshage. The two hex fittings are threaded on to a joiner fitting which I machined the hex off, just like the front brake lines.
Here’s the final product (I hope). You see, the tank is already in place so I had to use a welding rod to get the basic shape I needed, then transfer those bends to the stainless line. I put the joiner fitting in place so that the fitting can be installed and then hooked up to the vent tube, since there’s not enough room for the whole thing to twirl around as the vent is tightened.