Separate fuel lines is overkill. Inline 6 motors have brass elbows that feed the carbs. Simply remove the main jet cover on the base of the carb and see if fuel runs out when you pump primer bulb.
Mercs of that vintage have filters or screens in the carbs themselves. Later motors have stainless steel screens under the brass elbows. Earlier motors have banjo filters under the top cover of the carb (over the carb bowl cover). Both can clog.
The way you can tell if a carb (or cylinder) is firing is to pull the spark plug wire when the motor is idling. If the motor slows down or stalls, the cylinder is firing.
You can also inspect the spark plugs to see if they look the same as each other.
