There's no magic to an SU or Stromberg sidedraft carb and your penchant to chew things over is commendable. The side draft "variable venturi" Carb is so simple it can't really hide or overcome other engine malfunctions. So before these carbs can be adjusted it is imperative to go through the ignition and valve adjustment aspects of a tune-up.
That done you can turn to the carbs. You have called your carbs SU's so I would assume they are not Strombergs. As I mentioned in a previous post there are two types of SU's. The ones with an external jet and separate float chamber will have a choke mechanism on each carb--with the cable pulling a curved lever with an "choked idle speed" screw resting on the lever and a link to force the jet down. These carbs came on earlier models than the '71 but can be fitted and they will work.
The other type is probably what you have - with a float chamber as part of the lower half of the carb. Some models had chokes fitted to both carbs--some had only one. I suspect your original carbs had both choked--replaced by a set that only used one choke. Setting the idle speed screws has to be done with the carbs independent of one another. The rod links should be disconnected from the carbs. With the motor warmed you need to use a "meter" (such as a Uni-Syn tool) or a simple length of vacuum hose. With the tool covering the mouth of the each carb in turn you can then turn the idle speed screws to make the values equal. With a small diameter vacuum hose placed at the mouth of the carb and the other end in an ear--you can hear the suction and get the carbs "balanced" very well. That takes care of "balance". Next come synchronizing them. That means the pistons in the upper chambers lift evenly as the throttle plates open evenly. That gets adjusted be lengthening or shortening the two rod links. Blip the throttle and the pistons should rise an equal amount - and at the same time.
Mixture adjustment gets done by slightly recessed adjusting screws angled from each carb side. Turning them in richens the mixture -- turning out leans the mixture. When adjusting them for mixture you can use the small spring loaded lifting pin on each carb to lift the piston about an 1/8th inch. Lift the piston on the front carb and the idle should rise momentarily and then drop down. If the idle speeds up and stays there the rear carb has to be made leaner. If the idle just drops - the rear carb can be made richer. Then do the same with the rear carb--adjusting the front mixture. Sometimes that little pin is missing--a screwdriver blade can lift each piston instead. When the mixture is adjusted correctly the engine should rev up instantly when the throttle is suddenly opened--a lag or idle dropping too low could indicate too lean a mixture -- maintaining too high an idle speed after the throttle is released could indicate too rich a mixture. Be sure the dashpots have oil in them. -- Dave