Is it back firing just on tickover? Mine back fires if I've been giving it some beans then roll the throttle off
Only happens on warm up? Does it only happen with the choke on?
If you've just changed the plugs it could have significantly changed the spark/fire characteristics, meaning you don't need as much choke angle/duration as you are used to.I've recently changed my for the first time since owning the bike. Before the change I needed choke on for a couple of miles before I could take it off and not stall. With new plugs I can take it off after a couple of hundred yards.