Maybe because fuel blew out the carbies and it ran lean and detonated. Detonation is actually happening and doing damage even before it can be heard. If you can hear it, its already doing its work.

Most people beginning to experiment with blowthrough, always seem to overlook the fuel leaking from their carbie.
Its the most important thing. Think about how much fuel you saw leaking and then think about how much fuel a carbie actually supplies to an engine.
A really lean mixture still burns. It actually burns hotter than a stoich mixture. Being hotter it makes the engine produce more power and appear to be running excellent.
Lean mixtures turn spark plugs white. For something to heat up and turn white, it is really past hot and beginning to burn. Now think about the rest of the combustion chamber.

Dont over look your leaking carbies.