If you can, use a white permanet marker to make a line from the dampner hub to its outer ring. Your timing changes could be from the outer ring slipping on the rubber. Re-set your timing and keep an eye on the line for any changes down the road.
Have you checked the timing advance to see if its sticking. It could be stuck in an advanced position when you set the timing, then broke free and returned to the no advance position when you looked later?
The popping from the carb has always been a lean condition for me. Sounds alot like the secondaries aren't flowing enough fuel, but you need to get the timing squared away first before adjusting the carb. Maybe you can borrow a carb to test with? Worth a try before you blow $600.