Is it actual backfire, or just popping from fuel in exhaust? How is it when you're riding normally.

The backfire when you close the throttle results because the closed butterfly and slide has closed all the fuel flow except for the idle circuit...however the high vacuum caused by the closed throttle and revving engine causes a lot of air.......which results in the fuel air mixture being too lean to burn in the cylinder - but it can be ignited in the exhaust.  When the choke is "ON" it does richen the mixture a bit and there will be less backfire - however the mixture likely is too rich for good performance of the bike once warmed up.

The bike is primitive....it can backfire when you close the throttle.  You can reduce that by being more gentle closing the throttle.....close it slower and if the bike backfires when decelerating - just add a tiny amount of throttle and the noise will stop.

Weather was nice but snow was everywhere so I haven't chance to ride. What is the difficult between popping sound from exhaust and backfire?  When I heard the sound it doesn't sound like consistent for example I know consistent potato sound for Harley Davidson.
How about turn fuel mixture screw to more lean side it will help?
When I was cleaning carburetor I found out previous owner(I am 6th) already removed plug and set fuel mixture to rich position(I checked spark plug and found out tip and front side is all black so I replaced) as a now I think I set up mixture screw back to neutral position no hesitation but not high revving at neutral gear. Please let me know what you thinking.  Thank you.

The carb is subject to atmospheric conditions. My bike will after-fire (pop from the muffler) on a cool morning before a hot day. The rest of the time it doesn't. Look for a good average setting when you tune it.