Make sure it was rebuilt with genuine mikuni... Not cheap Amazon stuff. If it wasn't genuine mikuni, it won't run right and it'll probably burn up your engine. Just cleaning the carbs isn't good enough... at this age it needs a carb rebuild if it hasn't had one. What was your popoff pressure?
Some of the "genuine OEM" kits on Amazon aren't. Get the parts from a reputable dealer like osdparts.
After that, check and clean your rave valves. Make sure that they are clean and the black plastic caps that screw on to the rave valves are not stripped.
Next replace the o-ring in your water separator and check that the strainer in the water separator is clean. Remove any inline fuel filters that may have been added.
What speed are you hitting at 5k rpm? It should be 35-40 mph. Does pulling the choke a little make it run better or worse? You should check with the rpm gauge on the digital display--it can see minor rpm variations better than the analog dial.
