Does the unit have a coolant recovery system and have you checked it and the cap?
If it sucking air when it cools down because of an empty tank or bad hose, that could form a vacuum in the system which would pull in air.
If any part of the cooling system is higher than the fill cap that would cause the air bubble. A bad check valve in the cap could force it to suck air from the smallest leak.
Not sure if your genset was sourced from Wrico, but mine was from the PO, I have gotten great advice from Dick.
Is that small radiator worth "reworking", given labor rates it might be better to replace. (?) This comes reluctantly, from a very frugal engineer.
Just a thought, but likely most of the radiator shops are in the phone book or can be found online.
Likely someone in the area knows generators or RV's who could recommend the best radiator shop.
Generally, rapid overheat from cold is a telltale sign of a failing thermostat.
... If it were a head gasket one would expect it to show the problem continuously, not momentarily,
... Additionally, a head gasket leak would be leaking coolant into the oil or the cylinder, or both....
Quote... Additionally, a head gasket leak would be leaking coolant into the oil or the cylinder, or both....Not necessarily -- it depends on exactly where the gasket is damaged/leaking.
... If its not on the ground and isnt in the oil, its a pretty sure bet it went out the tail pipe. ...
... a call to Dick Wright about a Kubota engine problem would be a waste of time he just buys the complete engine he does no repairs doesn't have a clue except on his packages ...