KX 100 radiator question

You ever look at the impeller on the water pump??also look at you raidator mud packs in there and with a power washer you have to make sure you go from the backside and blow it forwards.Taking it off and soaking in soapie water you may find alot of packed dirt.Mix of anti frezze I mix water wetter as well

Also try distilled water with water wetter,
if the temps aren't near freezing.
Water transfers heat better than antifreeze.

If memory serves, water wetter reccommends no less 15% antifreeze. It contains chemicals to lube the water pump and anticorrosives as well.

You know those water pump impellars on little KXs just have a very smallscrew holding them on the shaft it could be loose . Also Ive seen KX 80/85/100s where the frame stretches and pulls the head studs out of the cylinder but the frame was also cracked at the area under the fuel tank . When you reinstall the cyl head did you have a hard time lining up the upperengine mount bolts? The lower bolts can be loose also , and the head holding the motor tight all by itself . Your head surface could be warped also.Just a thought as I have seen this before . GOOD LUCK