First you must bleed the trapped air.
Can start the bleeding with it shut off, and then finish with it running.
On right side of the bike, on the head just behind spark plug there's a bleed screw. Open it (unscrew) about 2-3 turns with the radiator cap off.
Squeeze the hoses a bit to move the water around. Watch for it to stop bubbling air out, then screw it back in finger tight.
Then start it up and open it again, just 1 turn open is fine. Watch for bubbling (air escaping) then when it stops and it's just water, tighten it up.
The other thing to check is the level in the radiator. When cold, and the bike level (on a work stand) only fill it 10mm over the fins inside the radiator to allow for expansion.
