Peter,
Not sure about the Ace SUs, but on other ones I have had the mixture is adjusted by turning a knob on the bottom of the carb body. If you reach under the carb, you should feel a knob that when you turn it, it move a sleeve that the needle is in. Clockwise rotation (when viewed from the top) will lower the sleeve and richen the mixture. There is also a a small rod that if you press it upwards, it will disable that carb by raising the piston so you can pin point which carb is not quite right. This is easy to do on a 2 carb system,but I am not sure how well it will work on a 3 carb system. I'll try and find an online link as I seem to remember there is a web site that talks about the mixture adjustment and probably will explain it much better than I have.
Jay