My liquid cool piaggo nrg is over heating what could it be ?

My liquid cooled nrg is overheating very quickly even over short distances it is fine untill it gets very hot and the throttle becomes less responsive , when the throttle is applied the sound is one continous sound instead of the usual sound , I believe theres a blockage I'n a coolant pipe but I'm really not sure any advice or ideas are welcome :) thanks

Jack - nrg
October 2011