"Backfire" out the exhaust, off throttle - any guidance?

[CO_GS500]

Greetings -

My '02 GS500 (4,300mi total, a few months and maybe 200mi since shop service incl valve adj) has just developed a new trick:
When coasting to a stop, as it comes under 3,500rpm it makes 3-4 loud, sharp 'smacks' out the exhaust, much like if you smack a towel on a kitchen counter.  Also tough to start all of a sudden.

It's always been cold hearted, but this is suddenly and significantly worse.  Runs wonderfully above 3,500rpm.

No mods, stock everything, haven't (yet) fiddled with the pilot screws, running mostly at around 6,000' ASL around Denver.

1) I'm planning a less chicken-hearted carb off, clean, replace job this week.  Anything that I should be aware of in particular?
2) My carbs are substantially different in design than those shown in the "M484-3" Clymer manual (which supposedly covers the '02.  Can anyone suggest a guide to carbs on an '02?
3) My airbox-carb-manifold bands are all cranked down tight, so I'm thinking about a light film of silicone when I put things back together.  Any big reasons not to do this?

Thanks for any suggestions!

[scratch]

Don't do the silicon.  It's just not needed.  Air leaking is not the problem, unless it's the airfilter; check that.

Did the shop do a carb synch after the valve adjustment?

Has the bike been sitting?

If so, how long?

[CO_GS500]

Thanks, Scratch.

I don't know if the shop did a carb synch - I see that it is not included in the list of 4,000mi svc items (unless it's a part of the valve adj).  Probe cropped up well after the visit.   Do synch problems show symptoms more at at low or high rpm?

Yep, I'd call 300 mi in 2-3 mos 'sitting' (it's very jealous of my DR650, and clearly hates having to share garage space with the Yellow Tractor).  However, it ran beautifully to and fro a trip to another shop for tires 2wks ago and once since.  My subjectove experience is that it's hyper-sensitive to old gas, meaning anything other than 1/2 tank very new.

It has occasionally shown reluctance to come down off 3,000+ rpm with the throttle shut, esp when cold, hence my fixation on eliminating any chance of airbox-carb-manifold air leaks inducing closed throttle lean. 

I've struggled and failed so far to find any online hint of what firing out the exhaust means...I thought that would be easy.