The plugged up ones that I have dealt with had hard to dig out black tar that was as hard as a carp. It was likely some kind of stop leak added several times by Farmer Bob, Otto, Norville and the like from back in the day to get another season out of an expensive to replace leaky radiator.
I cannot imagine any solvent that would break that stuff down to be able to flush it out. Engine removal and complete teardown is the only way that I could get it out.
