I had a valve nail a piston in my 440 before. It bent the valve and I found that by doing a compression test on it. I had 150 in a cylinders but one. I'll check he spark plugs and see if the one that went is not dirty as I changed all them righ after this happened cause I thought it was a bad plug. My wife did tell me it was backfiring and she pulled over and then it went away.