Triumph Repair: TR6 overheat, snap crackle pop, combustion gases

Question
started up my '72 TR6 for the first time in months and as it idled (about 5 minutes) the temp gauge went to the top limit (short of overtemp)and vaporized coolant was coming from the overflow hose . . . there was flow through the block (I checked by touch) and this has never happened before. . . carbs are Weber's and run smooth. . .timing was adjusted just before I parked the thing. . . .radiator fan was moving. . . any suggestions?  This is definitely the time of year to cruise in the car but I do not want to risk damaging the engine.  Thanks. . .

Answer
-Recheck the timing, and make sure it isn't too far advanced; the pulley on a TR6 is also a dampner with a rubber ring, and when the rubber ring fails, it can change the timing marks
-Verify coolant level; you should be able to idle the car with the radiator cap off to see when the thermostat opens
-Verify the thermostat is working
-Compression test to see if your compression is low; to be sure you don't have combustion gases entering the coolant

Did you hear the coolant boiling (snap crackle pop) when the engine was shut off?

It could also be a blocked radiator.