Would you be willing to weld two small bungs to the inside tunnel of your tank; one up by the steering head, and one somewhere at the back, accessible near the petcocks? If so, you could have a vent line at the top of the frame connected to the new bung via a short piece of rubber line, then do the same at the back of the frame and tank. Using my x-ray vision, I can see that, with the backbone sloped, it would find its level at the same as the tank, thus giving you the added volume without any valving and/or open, slit tubing.needing said:Frame tube fuel tank two-way vent valve criteria:
1a. Must allow air to escape when frame tube is filling from below, but
1b. Must not allow fuel surge to escape under heavy braking, and
2. Must allow air to enter as fuel is consumed from the frame tube, and
3. Must seal the fuel from escaping if, omnipresent hail Norton save us and protect us 'being' forbid, my bike is ever on its side on the ground, and
4. Small and compact enough to fit in the LH cup holder where the speedo went.