I relate that issue to my 40 years with Harleys. Customers would change exhaust and then experience that off idle "pop". Easier to tune out with efi tuners, but carb re-jetting and adjustments AFTER a good warm up did the trick. I also remember reading some years ago that Sox & Martin's engine guru the late Jake King used to change plugs (colder/hotter) rather than making jet changes to accommodate engine conditions. Good luck.
