Whats the ambient temperature? My best guess would be a fuel issue. When you start it cold the bystarter (choke) supplies more fuel for easy start up. After things are warmed up, the bystarter is not supplying the extra fuel, making it harder to start. Do you have the stock airbox and muffler? Have you checked for vaccuum leaks? Has the carburetur jets been cleaned? Thats the first things I would check. Im no expert, but there are certainly experts on this forum. Hopefully they can give some more insight.
