I think those have a screen in the carb end of the fuel line like the secret filter on the Triumph twins, this could be the problem. Also, trace the fuel line end to end, it's easy to get a kink in the line that is hard to see.
Also, check the running voltage at the hot side of the coil. Many Japanese bikes of that vintage have a shunt regulator that can't keep the alternator under control if the battery is weak and not pulling its share of current. This happened on the OL's training wheels bike, the extra voltage overheated the coils and killed the spark.
