This problem is somewhat inherent in a big single with a carburetor.....and is somewhat exaggerated by a CV carb that has both a throttle plate and a slide (and the somewhat lean factor settings don't help).

When you let the throttle close completely while decelerating or shifting gears....you are allowing the throttle plate to close and shut off all fuel flow, except for what can come through the idle circuit.  With the closed throttle plate the vacuum is very high and it pulls a lot of air past the throttle plate - but the fuel flow is very limited......the fuel/air mixture becomes so lean that the spark plug cannot ignite the mixture and it goes into the hot exhaust header unburned.  Once in the hot exhaust system the lean fuel/air mixture can ignite and create an explosion of the unburned fuel/air mixture.

Jetting for how a bike runs with a closed throttle is not desirable - you should be jetting for how the bike runs when the throttle is open.....accelerating/steady cruise/full throttle/etc.

If your bike is running a bit lean the backfiring could be improved by proper jetting.....I have no idea what jet sizes are needed in Newfoundland or what kind of fuel you have where you live (pure gasoline or mixed with Ethanol).  If the bike runs smoothly and you don't notice any lean surging - the bike may be jetted properly.  If the mixture screw is out 2 - 2.5 turns when you get the smoothest idle - then your pilot jet is likely the correct size.  If you can turn your mixture screw out more than 3 turns and the engine still idles smoothly - you could probably install 1 size larger pilot jet.  (NOTE:  When making the idle mixture adjustment you need to turn the idle speed down lower than normal....you will not be able to hear/feel the difference at the normal 1,100 rpm idle speed, as some fuel will be coming from the needle jet and make it impossible to make a proper idle fuel adjustment).

Turning the idle speed up a bit can help to reduce the amount of noise when coasting/shifting, as it allows more fuel to flow when you close the throttle.

You can also cut down on the noise and make the engine run smoother by adjusting your riding style.  When coasting you can open the throttle just a tiny bit....just enough to reduce or eliminate the backfire.  The same when shifting gear - just roll off the throttle a little bit between shifts and don't let the throttle completely close.

This is not a modern fuel injected bike - you need to help it to run smoothly and quietly.  When starting you may have to fiddle with the choke a bit and help the bike warm up.....and once warm you may need to adjust your riding style to help it run smoothly.  Trying to eliminate the backfire by making the fuel mixture richer may result in the bike running too rich when cruising.....and that can carbon up the engine, pollute the environment, and possibly shorten the engine life.