Refer to the following link for carburetor adjustments:
https://www.onlycubcadets.net/forum/...ad.php?t=58326
I'm not sure where you have your carburetors set, but it almost seems you have it set them all too lean which is often the cause of backfire. You mentioned you "started 1/4 turns" which is way too lean if I follow your description properly. Follow the table provided within for initial settings and hopefully you'll get your 1200 started.