Manually turning crankshaft produces a loud POP!
 

I'm doing my valve inspection. I'm turning the crankshaft... turning...turning, then suddenly a loud POP sound came out. Scared the crap out of me. Turned it some more and another POP. Sounds like a back fire, doesn't sound like something breaking. Is this alright?

Did you loosen/remove the CCT as the manual suggests? If so then your cam chain now has slack in it and once you turn it to a certain point the pressure from the valve springs will snap the chain forward and take that slack to the other side of the sprockets.

I wounded the CCT and put a stopper just like manual told me to. Did I mess something up?

so the tension is off the cam chain right? then you didnt do anything wrong. while turning the engine watch the cam chain and see if it jerks forward when you hear the pop