My Sportster Muffler is on it's way and I was wondering where I should go to get the jet kit I need to make the changes. This is something I have never done before but want to attempt it. I have an 2005 S40. It has had the spacer reduced and the mixture screw adjusted, but it still seems to backfire often. I had a local shop do the work. I am hoping that after the new exhaust is in and the carb is rejetted and readjusted I will be better off. Am I on the right track with this?

And you can usually get Mikuni jets at HD dealers.  You want the large round type.