I have a Lycoming IO360 a3b6d that has a bad backfire problem at idle. The engine has over 500 hrs since overhaul and has been doing this since being rebuilt. It is a new engine and airplane to me so I am trying to fix it. It has an airflow performance injection system, dual electronic EFii ignition and constant speed prop. It seems to be worse after flying, but if I lean it out it does not seem to pop as bad or as loud. I have looked and the intake system and have found no leaks. I was thinking maybe a sticky valve but compressions are in the high 70's and CHT's are in the green across the board. I have read on this forum before that RV's have a tendency to do that due to the exhaust system. I was not to sure about that. Thanks for any advice