HELLO
I was balancing the carbs on my nc35 recently and half way through this the bike cut off and there was bubbling through the expansion bottle . I immediately let it cool down and am trying to diagnose the problem.Heres what ive sussed out.

1.... the fan is working and cutting in when needed
2.... the water pump is working.
3.... there was no loss of coolant from the rad
4... the bike has no issues starting .
I am going to do a compression test on the engine soon but Iam wondering is there anything else I should do before this as regards the oil pump or thermostat.
any help appreciated!!
peavey

From the Haynes manual
Coolant level low
Leak in cooling system
Thermostat sticking open or closed
Faulty radiator cap
Coolant passages clogged
Water pump defective
Clogged radiator fins
Cooling fan or fan switch fault

I recon flush the engine put new coolant in and check the radiator cap