None of that sounds kosher to me. If you had/have carbon on the backside of the valve heads and the water did knock it loose (a possibility because GM used to sell a pour through the carb additive that was basically glycerine and distilled water designed to 'steam clean' the intake and exhaust to remove carbon deposits), how could a stray bit of carbon get lodged between the seating face and the seat itself and if it did, it should have just passed through after maybe a brief loss of compression.
The seating surface of the backside of the valve and the seat itself is narrow.
I'm curious.