New motor poping and back firing

Bought a new l-206. Briggs & Stratton straight from Briggs & Stratton and I can not get it to stop poping it's not lean , when I got it out of the box it had a crack in the head and I replaced that and still can't get it right this is not my first one. I rebuilt 20 or 30 of them in the last 6 months. If anybody's got any ideas I greatly appreciate it I've changed just about everything. You can change on it. Carburetor , coil everything