1994 Yamaha YZF750R Problem

I have had problems with this bike. This summer I tooked the old engine out and took it apart to see what the problem was. I end up finding out my valves were junk so it was cheaper for me to buy a used motor than pay to fix the valves on that one. So I swapped out the motor and I put everything back together. Now I am having a problem with it only running on choke and when I try turning it off it just dies off on me. When I start it, it starts right away. When I rev it, it seems to like not want to go so I need to turn the throttle slowly until it goes a little than I can rev it. Once it's somewhat warmed up it starts now come right back down once I rev it. It will stay at around 4,000 rpms. If anyone knows what is going on l'd greatly appreciated your help! Thanks.