Thanks for the responses everyone. That is useful info.

I have a set of these carbs being the BS34 models which I believe were fitted to a Yamaha XS1100 from the late 70's perhaps early 80's. What I have come to learn about these carbs is that from the factory on some models they set the two centre carbs a bit richer by fitting slightly fatter jets, say 115 on the outers and 120 on the inners. No doubt because they get less air to cool them and so run them a bit richer to try to keep the temperatures under control.This no doubt varied from manufacturer to manufacturer so any set you consider these issues should be kept in mind.

Thats all well and good on an air cooled engine but not a factor on anything that is watercooled. The needle heights are adjustable and have 5 different settings and so should be set the same on the bench before fitting up to the car.