There is a lot of discussion on the correct way to tune them. You are doing the hardest part right now. Once you get the settings down there is not much to them. If you try what is said in other forums and still have issues call the company and they will walk you through it.
It sounds like the engine is getting to much lp gas to the lp gas valve without seeing anything. Think a gas valve as a fuel pressure regulator(i do). If a gasoline carb is getting to much gasoline because of a bad regulator in line no amount of adjustment(with a few exceptions) will keep it from running rich and flooding/backfiring.
You can play around with the mixers all day long if you have a over sized or a lp gas valve that is turned up to high it will bog down. Some are adjustable others are not. The initial setup can be a headache and it will take a bit of fuel/try&adjust to get everything just right.