Have a 289 C-Code 2V a month ago adjusted static timming from 10 degree to 8 degree's BTDC this was done because I ran out of idle adjustment. Forgot about offsets to dwell so when I test drove it backfired thru carb under moderate load. Went home and rechecked dwell and was out of spec. Then reset at 27 degree's rechecked timming ,carb. float level and readjusted mixture screws to 2 turns out for max vacuum 19". Back out again all fine didnot drive for a month. took out today and backfired/coughed a couple of times then stopped. Fuel was down 10gals. so topped off with 90 octane and used my Lucas fuel stabilizer. Now only coughed once in 5 mile trip but cann't figure out? Carb was rebuilt 6 mos. ago new fuel filter etc. along with new plugs,points(OEM)rotor,condensor/vacuum advace it had been 15K since tuneup from previous owner. Read on line about weak coils/cracked dist. caps/stickey valve etc. could cause backfiring but havenot replaced yet. All this intermediate carb. coughing problem started after just 2 mos. ago. Have good stream of fuel from accelerator pump, could it be moisture in gas tank/bad gas since I was down to 5 gals? Sorry about the long post but wanted to give as much info as possible-Thanks Doug