TOPIC:

My Z650 has started to mis-fire and pop back through the exhaust when throttling down, so what is going to be the most likely cause, carbs, valve shims, condencers, or something else? she starts and tickover is sweet.

I would look at the simple stuff first. Check your ignition timing first with a strobe light. If you have a compression tester have a look for a bent valve. Vacuum gauges are a really handy tool to check carbs. Check any fuel filters for adequate flow.