I bought a 2000 RX last year and had a similar issue in trying to get it running.
First, make sure you've changed out the grey fuel lines if you haven't already.
Second, although the previous owner said the carbs were rebuilt, they may not have used the genuine Mikuni parts. I've read the cheaper parts don't last long. Also, when I rebuilt my carbs, I didn't change the needle and seats at first, and I had the same issue you're having. Once I changed those out, I was able to reach top speed.
If possible, contact the previous owner to determine what carb parts were used, and if the needle and seats were changed too. If they didn't use genuine parts, I'd build the rebuild kit from OSD parts and complete the process yourself. There are several carb rebuild videos on youtube to guide you.
Third, check your wear ring to determine if it needs to be replaced. Also plenty of videos to help you with that process as well. You'll need to build the impeller tool, but it's pretty cheap.
Good luck!
