I had backfires out of the front carb occassionally.
At first I thought it was because of my homemade linkage but I was able to rule that out.
After I had removed the carbs and intake again I somewhat recalled one of the carbs has been used as a test carb on a bad running engine and it got a few nasty backfires aswell there.
Maybe the throttleshaft is slightly bent or it has other issues. I haven't been able to find out anymore on this intake.
I hope I will know when it's installed on the 392 in the future.

The carbs were 625 Carters by the way.
Way too much for a 354ci but with the airvalved secondaries I reconned they still should work OK.