run a compression test .
Is it backfiring into the exhaust or carb , if it is in the carb that is usually lean condition , however if it instantly backfires the carb should still be full . You can disconnect the fuel line & see how much fuel gets pumped up out of the line at the carb , you should get a cup in 15 seconds or so . Suction is very inefficient so if the fuel lines are kinked it could easily be starving for fuel . What size tube are you using ?
Timing should be 14-16* at idle & 36*@ 3000.
are the ign wires tied together or seperated ? If the wires are in poor condition it is easy to have the spark jump wires if they are tied together . Any moisture under the dist cap ? Are you running Electronic ign ? If so check the gap between the reluctor & pickup , it should be .008 , I have seen engines run poorly just because the gap is too large.
Do you have adjustable valves ? If so is one adjusted too tight ?