It should not backfire at any rpm. I image revving it in P you are only opening the primaries. There is no real load on the engine in P, so the b/firing is probably sooty plugs. The plugs should be a 5 heat range in NGK, [ not colder if that is what you have been told ].
What ign system?
I do have sooty plugs I pulled them out and cleaned them the other day when I was checking the timing again I'll have to check the heat range they are champions