This is not the motorcycle that you should be closing the throttle quickly.  Doing so is certain to make noise when you do that, and putting on a muffler that is less restrictive than the stock one will make it even more pronounced.  Carb jetting might make it a bit better - but it will not stop it from happening - and you should jet to make your engine run well.....you should not be jetting your bike coast quietly.  If you jet the bike really rich in the pilot jet circuit you can cut back on the noise - it will make the engine run too rich at low speeds.    You need to adjust your riding style to suit the big, primitive single with a carb.....this is not a modern, fuel injected motorcycle - it is a primitive beast.

When you let the throttle snap close, you have created a high engine vacuum that pulls a lot of air past the butterfly and slide - but the main and needle jets are closed off....only the idle circuit is providing fuel.  The fuel mixture goes very lean as a result, and the spark plug can not ignite the mixture in the cylinder - but when the lean mixture builds up in the exhaust it can ignite and make the popping and banging sounds.  If you jet to eliminate the noise when coasting - the engine will be running too rich when at a normal idle (same thing when pulling out the choke).

When you shift gears you should gently roll the throttle off - but stop just a bit short of being all the way closed.  Same thing when you are coasting down a hill or slowing down in traffic - roll the throttle off just far enough to slow down - but not all the way closed or the noise will begin.

It is all part of being a "good" rider - you need to adapt your style to suit the motorcycle and it's quirks.

Leaner in the colder weather

Ok Norm that is info. I can use. Awesome.


Gus

Temperature, altitude and humidity will affect the carb settings. Less air = less gas; more air = more gas.  

Other than aftermarket mufflers and silencers; I wonder if anyone has experimented with header length at all?

Correct me if I'm wrong but header length only changes the power curve. It shouldn't affect backfiring, smoothness, etc.

My comment about the high vacuum was in a PM to Gus, and the condition I was referring to is when you close the throttle and the engine is slowing down with only the Pilot circuit providing fuel.

The fuel mixture goes really lean when the throttle is closed and the engine is either slowing down between shifts - or you are slowing down and have the throttle fully closed.  There is a high engine vacuum - but with the throttle closed and the slide down...the only fuel that is getting to the engine is coming from the idle circuit....and that is not enough fuel to allow the spark plug to ignite the mixture.  So the fuel/air in the cylinder is not ignited and it just builds up in the exhaust....and eventually the mixture reaches a high enough temperature in the hot exhaust pipe that it spontaneously ignites and makes a bang.

Suzuki tried to cure the problem with the TEV (Throttle Enrichment Valve) which is supposed to allow more fuel to flow during those high vacuum events - but it doesn't seem to work well with the E10 fuel.  Maybe it works OK for folks who can still find pure gasoline.  Some folks have fiddled with the spring in the TEV and claim to have made things better - but I don't feel it is a issue with the TEV not operating soon enough - I believe it just doesn't add enough E10 fuel.

There are folks who put in larger pilot jets and open the idle mixture screw, and this helps compensate for the lean mixture when decelerating.....at the expense of an overly rich mixture when the bike is idling or at low throttle settings.  Fuel mileage can suffer considerably, and the engine can belch black smoke when idling, and the engine can get loaded up with carbon.

You really can make a big difference in the amount of backfire by how you use your right hand.  When you shift gears just roll the throttle slightly back and don't let it close completely between shifts.  When coasting just open the throttle a tiny bit until the backfire goes away - but not so much that the bike is accelerating....just enough that the slide rises and allows more fuel into the engine.