Buying carbs off e-bay is a bit of a lottery and you could easily end up wasting your money (speaking from experience
). Most of the round tops are over 40 years old and unless you can either see/hear them running or have receipts for a full rebuild then you could have worn parts which, even with countless hours of tuning, will never run right if at all. The problem tends to be with worn throttle shaft bushes which will let in air. The way to check is to hold the throttle shaft and move it side to side and up/down to see if there is any movement. it should be minimal. The same with pulling it back and forth. It is an awkward job to replace bushes unless you know what you are doing because you have to drill them out very carefully.
Assuming that you throttle shaft is ok on both carbs, the first thing you need to do is set the float height. Take the lid of the float chamber and turn it upside down. bend the metal tang so that the gap between the float and the lid when it is sat on the float valve is the correct height (sorry I cant remember what that is but its in the Haynes manual). If the gap is wrong, the carb jet will flood or the chamber will overflow or it will starve of the jet of fuel. Either way it wont run. Then make sure the float valves are working by blowing through the inlet pipe whilst pushing the float up and down carefully. It should stop the air when you push it up
You will need to get some return springs if you dont have any as this will return the throttle and stop it over revving. They connect to the heatshield so if you don't have one, you need to find somthing that will allow the spring to have tension. A lack of heatshield may also cause the carbs to overheat so it would be best if you can get hold of one.
Assuming everthing is now in place, the next thing to do on the carbs is to wind the jet adjustment screw all the way in and then wind it back out around 2.5 turns on both carbs. This will give you a good starting base to tune the carbs. You will probably need to adjust the throttle stop screw or idle screw. Basically follow the advice in the earlier link but as it said, you will need an airflow meter.
Just a quick question, you say you changed the needles, have you got the right ones. For British SUs they are SM or DX needles but I don't think it is the same for Hitachis.