Hi reborne, good to see you taking an interest again, boy its took a long time to get over that Tomos ordeal
On your fuel tap, is there a prime position, if there is try that, you may find three positions, prime, reserve and normal.
fuel taps are often vacuum operated these days (thats another story). Prime position will run fuel down to the carbs directly even if the vacuum tap is not working properly, If it works ok in that position and then konks out in normal or res, then vacuum tap not working properly.
BTW if it is a vacuum tap you will have a fuel line and another line going to manifold somewhere