[tomw0]

Before declaring total disaster, I'd pull the head to see if the valves may be stuck. You had a big backfire out the exhaust, and it may have slammed the exhaust valve open to where it jammed wide open [more than usual, so it stuck on residual valve guide gum].
Can't hurt. This is the exact thing that caused the installation of solenoid operated valves on the bottom of float bowls.
tom