47-55.1 Exhaust manifold temp

[dsraven]

a thermostat and coolant is recommended because coolant has properties plain water doesn't as well as a thermostat restricts the coolant flow some so coolant has a chance to absorb heat from all areas about the same instead of cooling the higher easier flow areas better than some that flow less easily. also better for controlling heat during start up/warm up where the engine will take longer to get to operating temp, which saves the amount of overfuelling required at that temp, so less fuel washdown of the cylinder walls and pistons and also less oil dilution from that fuel getting past the pistons into the oil pan.
mr48 has a good idea to check the intake plenum under the carb to see if the hot cylinders are fed off the same carb bore. which may possibly be running leaner than the other bore(s). a spark plug color check and exhaust tip residue check may also prove the point to some degree.
if it were me I would do coolant pressure test and a rad hose/rad cap check and if that passes it would be followed by a chemical coolant flush followed by a good water flush to rid the system of anything loose. then allow to cool and dump the system completely. pull lower hose off and try to catch what you can so you can see what was in there. then pull the block drains and ensure the block is sediment free. maybe pull an expansion plug if you are not sure, pick a rear one if possible. flush the rad as well to ensure you have good flow from all the tubes if possible. if you run a heater, backflush that as well, removing the hoses of course so no sediment or debris from the heater can enter the engine. when you pull the spark plugs check for any super clean or rusty plugs as that can denote a cracked head.