Originally posted by KJ
they broke because of how long you were spraying. The intake valves have the cool air flowing over them due to the relatively cool intake charge (real cold since you were spraying) and the heat from the combustion cycle heats them up. This hot cold temperature cycle going on for a full minute will fatigue the intake valves until they break. I'd also bet some exhaust valves are also burnt and should all be checked and/or replaced. That is my theory anyways.
-werd-