Honda NX650 popping during deceleration

SOURCE: Suzuki C50 Boulevard (4k miles) backfires during

Backfiring when the throttle is closed shows a lean mixture. Either you, or your mechanic should take a look at the mixture, and there's a 99% chance that that was the cause of your problem :)

SOURCE: 80 xs 650 hesitation and exhaust "popping"

Most likely the carbs need cleaning and adjusting. You may want to richen the A/F ratio when you do it. They are set really lean from the factory. I would go 1/4-1/2 turn out on the A/F screw.

SOURCE: backfiring on deceleration from exhaust pipes

May be running too lean or too rich. Check the color of your plugs, black dry and sooty means they are rich, white with blisters lean.

SOURCE: have a 1989 vt1100 shadow--experiencing

I had that same noise, come to find out the horn was ever so slighty bouncing off my exhaust.

SOURCE: When I crank the engine

I suspect the float in the carb had stuck down.
Strip the carb down and replace any corroded jets and fuel float valve.
Change all fluids, oil etc. as you need fresh lubrication to keep the engine going smoothly.