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It is very easy to think a radiator is clean. I got caught some years ago when I had an F250 Ford 4x4 and it had a 7 lt mitsubushi diesel. It was overheatihg very easily, I flushed the radiator out and got some crud out but the problem persisted so out came the radiator and took it to a mate who was an expert. I said it seemed clean and flowed heaps with the garden hose upon which he had a laugh and took the tanks off and cleaned each tube. At least a handfull of crap came out of the tubes and more than half were totaly blocked. He said about 10 tubes will flow a garden hose and there were 108 tubes in my radiator.

Try to sort the ignition timing sorted out because an engine running even 6-8° retarded will overheat easily. The plates that hold the sensors will change timing by about 1°/mm so if you are 2 teeth out on the crank sprocket it could easily be 45° out with 16 teeth on that sprocket.