At a guess (total shot in the dark but it's happened before) you have connected the "auto fuel shut off petcock" vacuum supply line to the wrong nipple on the "auto fuel shut off petcock". The carbs are vented which reduces the chances that the float bowls will come under any kind of vacuum or vapor lock action. The attachment shows the correct line connections at the petcock.
It is common that most connect the #2 cylinder vacuum port to the petcock nipple which faces directly down. This is not correct. The nipple that faces to the side is the one to connect the small vacuum line to. It's also common that the vacuum connection gets completely forgotten about (does not get reconnected) in which case the same results present...no fuel flow to the carb float bowls. More pictures and words are located here: