once you have checked your getting a live to the coil , and are happy your getting a spark i`d check there is no water in the carb , then check the braided fuel lines that are near the filter, they rot inside , this makes the engine hunt for fuel ,this can give a very weak mixture, when the engine runs lean it can make the engine backfire like an ignition fault ,you can also get backfires when there`s water in the fuel , the fuel pipes look ok on the outside and perish on the inside , just a thought !