For fuel efficiency on multi carb setups, butterfly's open sequentially. For power, all butterfly's should be synched to open together.
Ergo, over time the plugs show rich on front cylinders. Change plugs, run 100 miles, and see how they look. Decide from there if balance needs to change. Beyond that, ID your carbs, buy balancers, learn how to adjust air/fuel mixture and synch and idles.
I am confused as to why your Z would have British carbs on it.
As for the power issue, check your fuel pump and lines, and rebuild the carbs. It sounds like it could be a fuel loss issue. SU carbs do require regular maintenance due to deposit build up. Granted, most people wouldn't think of ten years as a maintenance cycle.