And let's not forget that temperature sensors are electrical. Electrical on a 40 year old bike- likely corrosion = resistance= improper signals. Unless you've inspected every connection on the wiring harness and anything plugged in to the harness, I always suspect corrosion/faulty connections.
Would it be worthwhile purchasing an Electronic Infra Red Thermometer Gun ? They are cheap [ ish ] to buy now . You can point it between upper and lower Radiator Hoses , get an idea of Temperature rise , hot spots etc . Also a True reading , and not ''it appears to be very hot '' . I use one to balance my Central Heating Radiators . Just a Thought
I suppose a clogged radiator is unlikely?
What about a head gasket leak between combustion chamber and water jacket? Although that would result in not only overheating but also bubbles in , and loss of coolant due to excess pressure, I guess.
Take the radiator cap off and run the engine.
LOOK CLOSELY INSIDE YOUR RADIATOR............If your water pump is turning correctly you will see the obvious water flow through the radiator.
If its just still water and no obvious flow, then the pump is NOT turning so it could well be your Nylon Gear is shot.
The Nylon Gear can soften and wear very quickly or detach from its metal centre core if the engine gets too hot as its positioned in the middle of the engine where its very hot.
It very unlikey to be a head gasket as if coolant was geting into the combustion chamber you would see the steam results out of your exhaust pipe and it would run pretty crap too.
A defective Temp. sensor ? Certainly ! Another possibility - although unlikely - but because it hasn't been discussed: The voltage regulator for the fuel and temp. gauge. ? It's designed to supply about 7 volts to these two circuits. It's design is very simple and reliable. None the less: Check the voltage output. If it reads higher then the specs, this V/R could be the problem - giving you an erroneous High Temp. reading. The fuel gauge would also be a tell- tale sign by indicating an inaccurate (higher?) fuel level relative to what's known to actually be in the tank.
1980 KZ 1300 sr# KZT30A-009997
Always High - Know Fear !