912 ULS engine misfire/bump

Dont know what you mean by a "bump"

Backfiring is usually the result of :

  Faulty ignition timing

   Build up of contaminants (eg lead from AvGas) in the combustion chamber, causing pre ignition

   Valve(s) not seating

  Can also be air entering the exhaust system, - unlikly

You can also have these odd issues from a broken baffle inside the muffler.