Update: tapping on the valves with a brass punch on the worst cylinders (with the pistons at TDC firing) had no effect. I measured the height from the head of each valve to the spark plug seat with dial calipers before and after and there was no change. And no change in compression pressure. Oh well, it was worth a try.
It's unusual to bend valves on a engine especially as the usual causes like over-revving or cam timing slipping aren't applicable on a flatty. Why are these engines prone to it?
Malcolm
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