<VV> Popping back through carb

[4carbcorvair]

As far as I know, usually a pop or backfire back through  the carb is a lean
condition. Is there anything else that could cause this? I'm going to check
the filters. If they are fine, I'm going to pull the tops off and check for
crud in the carbs. Timing is honestly at about 14 or 15 degrees. Engine is a
fresh rebuild. Carb idle mixtures are about 1.75 turns out.

Thanks.

--
Ron Tinkham

66 Corsa Convertable, 140, 4sp.
Maine