\ Yes, the unknown,,,,internal damage from corrosion. A buddy of mine found a Royal Enfield 750 Mk11 Interceptor with frozen pistons. He poured transmission fluid in there and let it soak for a month. Then he rolled it down a hill and dropped it into 2nd gear and the 'unstuck'. Still running fine 10 yrs after, never taken apart. Then again, my brother went nuts on his 72 Norton,,,,full race cam, sodium cooled valves, balance, blueprint, Carillo rods, forged pistons. 3k later, it was a very angry 750 Combat that only wants aviation fuel.