Is there a fuel pressure regulator in the system, or built into the pump? How much pressure can the carburetors take before the needles will not seat and seal? is there a fuel pressure gauge? The fuel pump should not care about how many carburetors it is feeding. However, starting for the first time with only one carburetor (the others removed and blanked off) reduces the chance that the engine will be running excessively rich or lean, as there only two screws that need to be adjusted instead of six.
