Popping thru carb when starting

Mild 318, 600 elec eddy, cam? only pops when starting and that's only once in awhile. Pretty sure it's not carb, last summer was carb playing and had 3 different carbs on there and it did it with all 3. Car runs great going down the road it's just that occasional pop when starting. I'm thinking distributor and, or timing. running MSD with plugs, gapped 42-43.