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You have blown out the radiator fins and they are clear haven't you? If so, obviously your radiator is blocked in the core. try removing both hoses and flushing upwards first with a reasonably high volume but not too high pressure hose. When the water comes clear , pour water into the filler and it should come out of the bottom faster than you can pour it in. if it doesn't then you can try a radiator cleaning chemical, or maybe you will have to get it re-cored or replace it
how soon after start up does it boil?
knackered water pump can result in it boiling very quickly.
but most common is blocked rad , either internally so the water wont flow through or externally so air cant flow through the fins...you should be able to see through the rad from front to back.... valve adjustment will have little or no effect on water temp .until the tractor has been working for some time, even then its unlikely to cause actual boiling. if its starting and running ok dont waste time on valves.