Backfiring can be caused by a lot of things, exhaust leaks, valve lash, running lean, bad shutoff off valve and the list goes on and on.. In the last 1872 I purchase the M18 seem to starve for fuel like your if I push the throttle up, I did the K.I.S.S things first AKA Air filter/spark plug/gas filter/ clean the carb and adjust the mixture and the idle screw by the book. Better but still struggle to go up in the RPM, I swap a gas pump from a spare M18 I have and adjust the mixture and the idle screw again, Problem solved! Since that M18 never backfire I didn't look further but you may have to.
I'm surprise no one shim in to help you, maybe you get better result if you post that question in the engine section?