Last month I bought a used 2006 S40 and really enjoy the bike.   Right after buying I thought is was running lean and came across this great site that confirmed my belief.    It seemed simple, and I also found effective, to remove the brass plug from the mixture screw.  A 1/2 turn dramatically reduced low throttle hesitation and flat spots but seemed to increase backfire at 1/4 to 1/2 throttle.  Yes, my next step is to tackle the infamous spacer.   If that fails I replacing jets is the next step.  
 Sorry about being long winded but I an finally getting to my question.  In reviewing internet sites it appears Mikuni produces many styles of pilot and main jets.  Which are the right ones to purchase by product number?   On one site I found 4 styles of Mikuni main jets (M10/small hex, n100 large round, N102 small round, etc).  The same is true for the pilot jets.  Reviewed many postings on this site and could not find the answer.  Since I am too lazy to keep looking or remove the jets I thought it best to ask the expers.
 Thanks for the great information found on this site.  Not only will it be fun to tinker with the bike I will save enought money to buy accessories without the wrath of the significant other.  Or at least that's what I hope.

Also read the "no more backfire" in this site. Love the way mine runs now and notice no blue-ing on header pipe from too lean a condition. I'm saying take it all the way out and try. You can always undo it (takes me less than1/2 hour)