Dear BonnieCC: I too have an 06 s 40 and just recently put on a Harley Sporster muffler and did the rejetting. I had to search for mine but finally found it from one of those fly by night motorcycle shops--they had one lying around and sold it for cheap. Go to Topic: Sportster muff installation (Read 3995 times) under Technical Documents forum. That has detailed instructions on how to get the muffler on. Here is a picture of mine:
http://w2.bikepics.com/pics/2007/03/27/bikepics-843565-full.jpgIt sounds deeper and is only a bit louder. It didn't run very well until I did the carb modifications but ran ok with just the spacer modification. I bought the dynojet rejetting kit b.c. it has all the intructions there and did that and now it runs awesome, no backfiring, and cruises at speeds more than acceptable for interstate and highway use.
Do you want to do the mods yourself or get your mechanic to do them? They do take a lot of work (finding the right parts too) and concentration but I really enjoyed them. I also must say there is some risk involved, in that its possible to get yourself into some problems and then you have to think how to solve them. The carb parts are small and delicate and require care in getting them off (those pesky brass screws) and on again. My wife and 3 year old got a bit annoyed with all the time I spent with the bike but now it is a far superior machine and I am much happier with it--you realize how inferior the stock one really is. I am saying this as a Newbie who never worked on a motorcycle before getting my s-40 Savage and its great to be able to work on it as well as ride it.
I think that the white spacer modification solves the backfiring for the most part but you also might have to adjust the fuel/air screw (involves drilling out a plug) a bit to get it just right. The main jet is underneath the carb (that took me a while to figure out) but you get to the white spacer from the top.
Thanks to the guys on this website for their great advice and instructions. I am sure the future will bring me back asking for help!