What do you think happened?

Sorry for not responding. You are all on the right track, something slipped/opened up. I haven't been able to really look at it much here, cold, snowing, lazy. I did pull the plugs and they are black, I've been checking them all summer since we rebuilt the engine last winter. They've been nice light brown/gray all year until that last road trip. It's running really rich. I need to warm it up and do a complete check of the timing, vacuum, etc. Then I'll have a better idea of what's going on with it. Thanks for all the suggestions.