I agree it might be a hung valve.
A few years ago I installed a running engine toward the end of a season, put it in storage for the winter, went to fire it up the following spring and had backfires coming through the carb.
After some fumbling around a friend and I used a borescope and discovered a valve was stuck open. It was bad enough to where a pushrod was bent. After removing the heads (because if I'm rebuilding one, might as well rebuild both) I was met with the surprise of a valve that needed to be pounded out of the guide.
No idea why or how it happened but I went from a running engine to a head rebuild just like that.
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