Spitting/Backfireing/Popping through carb

[mad68rs]

I just got my camaro running, it's a 396 bored over with dual 4 barrels. They are both 600 cfm edelbrock 1406. It starts up just fine, runs fine but i get an occasional pop through the carburetors... it seems to happen randomly. It seems to do it more when it's cooler outside so i think I might be running lean... Timing and valves are good (12 degrees inital 33 degrees total)... new everything as far as ignition system (MSD distributor, blaster ss2 coil)... i've already checked all those things. Any ideas or help would be great. Thank you.