I just put a DJ jet kit in my girlfriend's S40.

Bike was 100% stock. In very good tune - I adjusted the valves, decomp, new air filter, and put in a new plug before I played with the carb. It would backfire very often and with a loud backfire when shut off. Fuel screw had very little effect. Bike would still idle with it turned all the way in.

I went to a 47.5 non-bleed pilot. (I wanted a 50 but the 47.5 is still better than the 52.5 that was in it.) I installed the jets recommended by DynoJet. DJ140 main, DJ needle on the 4th clip.

Fuel screw 2 turns out and now it responds the way it should to adjustment. It rarely backfires when riding and just has a light "poof" when you shut it off. When it does backfire it is much quieter than it was.  Bike runs great and even starts quicker.

I'm sure this has already been discussed on here somewhere, but I'm new and like to learn the hard way. The taper on the DynoJet jet needle is almost identical to the stock jet needle. Shaving down the plastic spacer would have had the same effect as installing the new needle. I saw a member selling jet kits. I think it was Lancer. I should have bought one of his and save some cash.

PS - I'm at about 900' elevation.

Mine rumbles & grumbles & sounds great, I think. Every once in a while itll give me a decent sized POW, but generally, it sounds like an old truck w/ a  glass pack & standard trans on decel,, just a low grumble with a few minor exclamation points.

Mine did now and then. At times on shut down it had a lil bit of a pow yet  not very loud.
She has not had anything done to her jets yet nor her valves. That may change soon.  

The poll doesn't have the choice for mine. It has never yet popped on shutdown, and only sometimes does it pop on deceleration. Exactly the same as my old Suzuki Tempter did, back in '83.

I love it when it rumbles!  I certainly wouldn't want to "fix" it.

Just a poof on shutoff. I can create a backfire by holding in the clutch and revving it up a few times while riding  then putting it back in gear...or sitting still in"N".
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Edit*
Update: based on what sjaskow has said
I do only use  93... Primarily from Shell
93 = a kind of sigh on shutoff and no pops on decel

I get a poof on shutdown and the rumbles on decel, with kerpows when it goes down to between 3,000-3,200 rpm. This RPM range is almost guaranteed to create a kerpow, not above and not below that. I don't know why it is like that. At night you can see blue flames coming out of the pipe in unison with the rumbling, and when the ~3,000 rpm kerpow occurs, there's a blue flame coming out about 6 inches long. It constantly changes during the day on what it does. I can have days where it doesn't kerpow at all, no matter what. Next day it's back with a vengance. Some days there's not even the poof on shutdown. And I didn't change anything at all.