I’m struggling with the same thing though mine only erupts when I open it up to approx 60 mph and above. In my case it seems a smaller than normal non-honeycomb radiator is responsible as rad and engine have been well flushed and the engine is no longer running lean. It takes a lot of energy to boil water so the engine is still being cooled but obviously only while the water level remains at reasonable levels 😯
If your bike doesn’t boil when given some “beans” then your radiator must be working fine.
Keith
