Things I'd check:
1. Water pump - make sure it's actually working. Remove a hose to see what the flow is (heater hose is good). Take care - the water will be very hot and only do it for a second or two but shutting engine off.
2. Air lock - you may have an airlock in the system that needs bleeding out.
3. Compression test - may show any headgasket problems.
"Y mae dafad ddu ym mhob praidd"
1972 4/4, 1981 MGBGT, 1984 FLHT1340, 1980 XLH1000, 1990 ZX10
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