The mixture adjustment on your HIF (horizontal integral float) S.U. carburetters can be found just below and offset from the mouth of each carb. The front one is to the front and the rear one is to the rear. It will appear as a protrusion of the body's casting with a hole in it (about 5/16') .You must have the air cleaners off to make any sense of this. If you look down the bore of this hole you'll see a crossed set of screwdriver slots, not to be mistaken for a phillips head screw. They are a bit of a pain to access and I long ago built a special screwdriver out of a standard stubby to make the adjustment easier on my customer's cars. Clockwise richens the mixture and anti-clockwise weakens it.
A note of caution is in order. If this car has not had carburetter attention in a long time, it is possible that it has old jets which are susceptible to what we've always called "stretched jet syndrome". The jet is composed of a brass tube molded into a plastic holder which is in turn positioned by the mixture screw via a bi-metalic bellcrank affair invented by satan. Sometimes the brass tube would begin to pull up out of the plastic holder causing incubable leanness. To check for this, screw both jets all the way rich and see if one (perhaps the one you mention as going lean) won't go nearly as far as the other. If this is your problem, don't mess around. Put a rebuild kit into both carburetters and replace both jets. While you're at it take note that the floats in these babies are well known for sinking. Not a great thing for something called a FLOAT! You may gather that I'm not a big fan of HIFs. Hope this helps
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon