chevy 327 popping at low rpm??

ok so i have an 81 camaro with 327 and m21 4-spd, motor has been in for about 6 months but havent been able to drive yet due to winter and money. Well the car has been running awsome for past couple months. I randomlly start it and let it run for a while and put her back away. Yesterday i decided to pull my batt out and put in my buddies truck. when i put the battery back in and started my car it ran rough,and started popping out of the exhaust. sometimes backfiring,would idle right, and when i throttle up it seems like an awful misfire. The popping is very constant at low rpms and goes away the higher the revs get. Now this happened out of the blue! I know the battery is unrelated, both terminals are fine ive checked 100 times. timing is still dead on, wires are fine, i removed checked and replaced multiple times, ignition wire for dist is good with 12 volts, and nothing was touched on the carb. It really bothers me how its running fine one minute and a hlaf hour later it wont even stay running for me. Could exhaust valves be a consideratioin? maybe a valve backed off?? idk!!! i havent checked heads yet but im at work and cant stop dwelling on this!!! can anyone throw out some suggestions??