These vacThese vacuum petcock/fuel valve on carbureted HD models can be rpone to devoping a pin hold in the diaphragm on the back of the fuel valve, when this happens it shuts off fuel to the carb. A quck roadside fix is to disconnect the vaccuum line, loosen the fuel valve swing it around so you can get to the screws on the back of th valve, remove the screws and the diaphragm body, don't lose the spring, install the spring on the opposidte side of where it was, this holds the valve open, turning your valve into a full flowing valve with no shutoff other than by turning eh fuel selector to on or reserve. Stick fuael valve back to its original position and tighten the nut securing it to the tank,don't forget to install the vacuum line back onto the valve or you will have a wicked intake lekak.