Well gents (and ladies?), here you go. Lost a valve seat in the #3 exhaust. I’m so relieved to finally solve this! I’m also relieved to know that it wasn’t someone messing with my ride.
Always good to find the real cause!
IMHO, I would think twice before re-using those head. Yes, they have been gone through, but obviously not in a quality fashion. No telling if all the insert work is marginal or not; look at the big step under the seat in #7. And there is a lot of erosion/rust pitting in the throat under the #3 exhaust seat. Not suggesting you make big upgrades just for the sake of more HP and torque (and since the rear axle may not take it).... just for getting good, reliable parts in the engine.