Many people in this forum have many different types of jet kits, I have an 07 s40, changed to dyna exhaust, bought performance filter, its only a piece of white foam stuff, just a square of foam. Havent put that in yet, rather have a K & N. what size jets are best with dyna exhaust.

Really curious how you plan to use the S40 - if you plan to ride it a good bit - say 3k miles a year or more,    
If it were me, first I'd toss the "performance" air cleaner in the trash and think about where I ride - if it's dusty at all no K&N either - really isn't a single jet that controls it all - if you read up on the white spacer you'll discover there are three part's - the idle mixture (that is covered with a brass plug) alng with the pilot jet this handles the lowest throttle , the needle jet this controls the midrange and the white spacer sets the rich/lean for this, and finally the main jet - used mostly for wide open throttle - stock the main is a 145 - going to a 150 or 152.5 is where I'd start - but you need to work over the white spacer and the idle mixture as well to tune this bike.

we have a straight through harley pipe(muffler?) we live at 300 ft above sea level, and found our combination to work very well for us. the air filter is stock, we live in the desert with lots of dirt, better filtering is more important than big air flow. the air box door has been swiss cheesed, that would be (12) 3/8" holes drilled into the pre-filter side of the door, to improve air flow. some folks remove the air horn on the top of the air box to improve air flow, i have wiring there from the sportster seat mod. i run a 152.5 main jet, 1/2 spacer, and 55 pilot jet. the exhaust runs just a little bit black(fat), the plug is normally a milk chocolate brown, and we're getting between 40-50 mpg. your tuning may need to be different, depending on where you live/ride.

If you have4 enough fire to be a serious threat to things, you have a serious enough issue that NOT rejetting will do more damage than you can do by goofing up trying. About the worse you can manage by rejetting is get it too rich or leave it toolean, But, it will be closer than it is now. Have ya got a GOOD phillips screwdriver, #2 head? Cuz ya gonna need it. If you have a friend who can help hold the carb & you can grab your screwdriver with pliers & hold it very, very tight into the head of the screw & give it a quick twist you miught just snap those too tight & dry screws right on out. Popping the screwdriver a few times with something, driving it into the screw will tend to shock it & get it ready to come out, may even be worth the trouble of trying to get a penetrating oil in it first. IF you do booger a head, you can use a small punch & light hammer & drive the meat back where it came from, do it slowly, several light taps are better than one Bigggun. An impact driver can be very helpful. I used antisieze when I went back together. There is a pretty good chance you wont nail it on the first pass. It's a trial & error thing, with some general guidelines to get started. It's important to be careful to not let the screwdriver messup the jet & make it hard to get out, be careful & take your time. It's really not technically difficult. Pay attention, place parts in a logical order to make going back together less of a puzzle. Not the plate in the bottom of the slide has a dimple & that it points down, else the holes don't line up well at all. ( Dont ask,, Okay?) A magnetic screwdriver is pretty handy. I can't remember if those little screws in the base of the slide are #0, #1 or #2 phillips, but a magnetic screwdriver with interchangeable bits will come with all 3. Looking at how it sits in the screw & how it fills the slot will tell you if you have the right one. Look at how the #2 fits the bigger screws & then you will know how it needs to fit the smaller ones,, flashlight, maybe?
Clean, clean, clean, no fingerprints on the slide. Watch the tab on the diaphragm, it will show how the slide is oriented.

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???  Maybe you've explained this before but....details?  This doesn't sound good, but may be no big deal, what happened?  How do you know this?  Did you catch something on fire?

I have a dirt bike that is tuned for very hard riding.  Flames will shoot out the back of that bike under certain conditions at WOT followed by snapping the throttle shut for a quick shift then back to WOT, but that thing is tuned pretty rich.  Mine is NOT a dangerous condition...as long as nobody is directly behind me

I dont have flames enough to catch anything on fire...I put the exhaust on a week ago...it was smoking a lil at the clamp so I asked on here what that meant, they said tighten the clamp...I tightened the clamp as tight as I could..by hand... no more smoking...I took it for a ride around the subdivision last nigh and I pulled up in my drive around dusk and I saw a flash of light behind  me as I revved it up....I revved again and watched the muff and and a flame shot out as it backfired....not a FLAME as in a cig lighter flame but a burst of fire like a torch flame.. only lasted a second, and only did it when it backfired...