what caused all this?

[66StangGuy]

Recently installed an electric fan so i took the old one off and re installed the pulley. Also put 91 octane gas in the car. re adjusted the timing and everything.. ran ok for a while..i think i had bad gas. anyway ran it out and bought 87 again. reset the timing for it and have ben running that. now for some reason i get a popping and banging out the exhause most when its cold but still occurs when the car is warm. With out the old fan the car does run warmer. could this be causing it? used to run at 160 all the time hot or cold out. cooling worked great. now with the electric it runs at around 180 depends if im driving or not. what causes this poping and banging and i though it was an ignition thing but i dont think so.. iv checked everything and i have magnetic pickup so dont think its that. iv checked everything else that would be ovious. ill try new plugs and wires this weekend but i still dought thats it. any ideas to try. the poping is only out the exhause and only around 2000 - 3000 rpm. checked the carb and its fine. someone have ideas? i cant think of anything else..
ill keep messing with it this week and let you know what i come up with.